The letter to Equalities Minister Liz Truss was written in response to a consultation on outlawing the "potentially harmful" practice.
It is signed by more than 2,500 ministers and pastoral workers, and says banning it would breach their legal rights.
But Robert Mee, founder of Out in the Bay, said he was "shocked" by the letter's support by local church leaders and said it felt like "society seems to be regressing".
One Morecambe web firm also said it had "severed all ties" with one of the churches listed, and no longer hosted its website.
According to NHS England, conversion therapy tries to change someone's sexual orientation or gender identity.
It warns that all forms of the practice are "unethical and potentially harmful".
In October, the government opened a public consultation on its plans to make administering "talking therapy" for under 18s or non-consenting adults illegal.
But the church ministers' letter describes conversion therapy as a category "so broad as to be essentially meaningless" and legislation to ban it "strongly risks" criminalising them.
The letter, signed by 2,546 Christian ministers and pastoral workers, was initially sent to the Secretary of State in December after a public consultation was extended until February.
Around 5,000 members of the public have since added their names in support of the letter.
Among those to put their names to the letter are:
*Rev Jeffrey Haskins, Pastor, Grace Baptist Church, Lune Street, Lancaster
*Rev Danny Rurlander, Senior Pastor, Moorlands Evangelical Church, Middle Street, Lancaster
*Rev John Mosey, Retired Minister, New Life Church, Lancaster
*Rev Marcus Mosey, Minister/Pastor, New Life Church, Lancaster
*Rev Steven Hewitt, Pastor, Stanley Road Baptist Church, Morecambe
*Tom Day, Minister-In-Training, Church By The Bay, Balmoral Road, Morecambe
*Benjamin Petersen, Assistant Pastor, Church By The Bay, Morecambe.
The letter says: "We are Christian ministers and pastoral workers from a broad range of churches, who have in common that we hold to orthodox, historic Christian teaching on sexual ethics.
"In our churches we welcome and show love to many people who have different experience and views, including same-sex attraction and forms of gender transition. We always seek to act in love, with gentleness and respect, for the good of all, and never with any form of coercion or control.
"We are grateful to the government for having made clear in the consultation that it does not intend this legislation to impact the normal practice of religion. Nevertheless, we are deeply concerned that the legislative approach outlined would do exactly this.
"We see in these proposals a clear possibility that our duty as ministers, of proclaiming the Lordship of Jesus Christ, and calling people to find life in him, which includes living by his laws, will be criminalised. We also believe it could be used against Christian parents who could equally be criminalised for loving advice and teaching given to their own children.
"The category of ‘Conversion Therapy’ is one which is so broad as to be essentially meaningless. It has the effect of implying an equivalence between calling people to conversion to Christ, which is our duty as Christian ministers, and evil and disreputable past practices which are already illegal and which Christians are the first to condemn.
"Legislating against such a bizarrely broad category is clearly not viable and strongly risks criminalising us as we fulfil our compassionate duties as Christian ministers and pastors. This would be a clear breach of our legal right to manifest our religion.
"Christianity has always held that God created humanity with the lifelong marriage of one man and one woman as a gracious gift to humanity and a central part of his design for human society. To violate that pattern, by sexual activity outside marriage or denial of our created sex, is sin.
"As such it is not only morally wrong but carries with it deep and tragic consequences for individuals, families and society. It is a central part of our calling to bring Christ’s compassion to a broken world, that we call people to live according to God’s gift and pattern of marriage and offer them pastoral support to help them do so. This has nothing to do with therapy; it has everything to do with what it means to be a Christian.
"To urge and assist people to live in this way, far from being harmful, is a kind and merciful act, and of benefit to all. What is plainly and terribly harmful is when anyone, especially the young, believes that their identity is found purely in their feelings and that happiness is to be found in misusing and harming their healthy bodies. Yet the proposals would apparently criminalise us for seeking to care for people and seeking to dissuade them from this kind of harm.
"Furthermore, it is our duty to call people to be converted to Christ; that is, to turn from believing that we are identified and best guided by self, to knowing that we are identified and best guided by God. Christian Conversion inevitably means giving up on lifestyles incompatible with being a Christian, of which sexual sins are just one.
"It should not be a criminal offence for us as Christian ministers to persuade, to teach and to help people of every age to become, and to live as, orthodox Christians.
"It should not be a criminal offence for us to instruct our children that God made them male and female, in his image, and has reserved sex for the marriage of one man and one woman. Yet this seems to be the likely outcome of the proposed legislation."
On the discussion of the issue on the government website, it says: "The government will ban conversion therapy. There is no justification for these coercive and abhorrent practices and the evidence is clear that it does not work: it does not change a person from being LGBT and can cause long lasting damage to those who go through it.
"We are committed to building a society in which conversion therapy no longer takes place. Our intention is to bring forward a ban in the criminal law that is supported by additional civil interventions that will ensure these practices are ended.
"Our approach has been built on a detailed assessment of the existing legislative framework to identify gaps that currently allow conversion therapy to continue. Having identified gaps in the law, we have developed a practical package of civil and criminal measures to put a stop to these coercive and abhorrent practices. We are determined to deliver a conversion therapy ban that protects people from these practices."
Robert Mee, chief executive officer at Out in the Bay, a Lancaster-based charity which works with LGBTQI people and young adults, people who misuse drugs and alcohol, and those living with long-term sexual health conditions, said: "Personally I am shocked that churches would sign to support conversion therapy, it is outrageous in 2022 - society seems to be regressing.
"People are still attacking LGBTQI people and they are getting away with it - the attacks are more discreet and secretive and almost whispers, but they are still attacks on us - have we forgotten about the holocaust and how LGBTQI people were persecuted and killed - history is repeating itself.
"Conversion therapy is barbaric - punishing people for being who they are. Surely churches are supposed to love everyone - did God not teach this?"
As a result of the signed letter, Torrix, a small website hosting business in Morecambe, announced they had taken the decision to permanently suspend their hosting for Stanley Road Baptist Church.
Torrix proprietor Matt Fletcher said: "I am an ally to the LGBT community, and endorse an outright ban on conversion therapy worldwide. I therefore cannot support any organisation that seeks to perpetuate the abuse of LGBT and/or non-binary people. Therefore, I have decided to withdraw my hosting provision for this website with immediate effect."
The website previously hosted by Torrix, and pointed to by the domain stanleyroad.org.uk, has now been suspended, with a notice on the Torrix server detailing the reasons. All links between Stanley Road Baptist Church and Torrix have been immediately severed.
Although the website hosting had been provided free of charge, Torrix had made a 'modest' profit on domain name renewal fees. A donation for the same amount will now be made by Torrix to a local LGBT charity or organisation.
A group of Christian leaders have also spoken out to support the ban, and have written to the Secretary of State to express their concerns about the harm "caused to many by the insensitive and ill-informed nature" of the ministers' letter.
"The pain that many have experienced in seeing their priest or minister, or indeed one local to them, assent to these views ought not to be underestimated," they said.
"We reject the concept of one Christian view on human sexuality and gender identity, as well as one Christian view on conversion therapy. Our affirmation of lesbian, gay, bisexual, transgender, intersex, and queer identities is rooted in a breadth and depth of Christian theology; as is, therefore, our heartfelt and informed desire to see an end to conversion therapy in the United Kingdom, and around the world.
"It is our belief that conversion therapy is indeed clearly defined as “...an umbrella term for a therapeutic approach, or any model or individual viewpoint that demonstrates an assumption that any sexual orientation or gender identity is inherently preferable to any other, and which attempts to bring about a change of sexual orientation or gender identity or seeks to suppress an individual’s expression of sexual orientation or gender identity on that basis.”
"We also all believe that conversion therapy is coercive, and that therefore informed consent is not possible. It is important to distinguish between genuine choice, and that which is coerced by virtue or culture, the interpretation of religious teachings, and context.
"We believe that lesbian, gay, bisexual, transgender, intersex, and queer peoples are made fully and beautifully in the image of God, and that their sexualities and gender identities can neither be cured nor changed – for this is how God has made them to be.
"It is this belief that motivates us to encourage and remind all those who have suffered at the hands of the Christian Church, and all those who continue to suffer today: that you are loved and valued, and that these undersigned clergy, ministers, and Christian leaders, do not wish to see you harmed.
"We will continue to pursue a Church and a world that truly represents the love of God as expressed through the life, death, and resurrection of Jesus Christ."
The local church leaders who signed the letter made the following joint statement: "We signed the letter, alongside more than 2,500 ministers, in a personal capacity because of our concerns for this proposed law's potential impact.
"As the letter says, we would join with Christians across the country in describing some historic practices in this area as abuse so it is misleading to say we are against a ban on conversion therapy.
"However, it is vitally important that the Government is clear on what is to be defined as ‘conversion therapy.’ We are concerned it may be interpreted to include the teaching of traditional, orthodox Christian beliefs on marriage and relationships.
"This could have a significant impact on our day-to-day ministry of teaching the good news of Jesus Christ as outlined in the Bible, which we think is both wonderful but also challenges every person."
In response to the action taking by Matt Fletcher, Rev Steven Hewitt, pastor of Stanley Road Baptist Church, said: "I am saddened at this development, but I am grateful for all the help that Matthew has given the church previously, often voluntarily."