As the Guardian reported last month, plans for a mammoth transformation are set to be discussed by city councillors later this summer.
Up to 9,185 new homes are expected to be built in south Lancaster over the next 25 years, many of them as part of a new Bailrigg Garden Village.
This could potentially see around 30,000 new residents in a city which currently has a population of around 53,000 - within a district of 148,000.
"The size of the final development is far larger than we originally expected," Coun Jackson said.
"It would take some time to set up the necessary infrastructure for a development of that size."
And a group of retired GPs have now raised their concerns about the plans - saying the current NHS system is already struggling to cope, and would be at huge risk with so many more residents.
Dr Averil McClelland and Dr Robin Jackson - along with fellow former GPs David Elliott, Peter Nightingale, Andy Gallagher and Paul Tynan - have joined forces to contact every Lancaster city councillor, as well as Lancaster MP Cat Smith and Morecambe & Lunesdale MP David Morris.
The proposals had been due to be discussed at full council on July 28, but this has now been put back until next month at the earliest.
The GPs are urging the councillors to look closely at the scheme and how it would impact on medical services in the district.
An estimated 30,000 new residents would equate to 15 full-time GPs in a city which is already struggling to recruit new doctors.
In addition, other healthcare professionals and clinical staff at the surgeries would be required alongside GPs.
Dr Jackson said: "What are the healthcare repercussions of this? We need key information, such as where are the people coming from - within the city already or outside - and what's the planning schedule for this?
"They need that information so that the councillors could be assured that the healthcare services will be available.
"Our biggest concern is that nobody is putting that information forward, and we are concerned it will be an unstoppable process and the NHS will be left carrying the can.
"We already know that the NHS in Lancaster can't cope with the number of people it's got."
Dr Jackson added that plans for a new 'super hospital' between Lancaster and Preston might not account for any increased number of residents.
He said: "This is a huge risk and that cannot be overstated given the difficulties we have got at the moment. Are councillors prepared to put their constituents at risk in what currently would be an uninformed decision?"
Dr McClelland said anecdotal evidence from former colleagues points to a worrying trend.
"A lot of GPs are leaving the profession before retirement age or in terms of the amount of time they are working," she said.
"There's already a crisis so what's it going to be like in three or five years with all these new people? Healthcare demands are increasing, and if they are struggling now we are going to be even worse in five to 10 years' time because demand will rise and the struggle to recruit is increasing.
"It takes 10 years to train a GP and, having closed to EU doctors, where are they going to come from? We have got fewer GPs and more work for them to do.
"As retired GPs we are seeing this as a big crisis and it's going to get worse. Our former colleagues are all completely worn out and desperate."
Currently 14.1 per cent of UK GPs are qualified overseas, and Dr Jackson said the NHS "relies on poaching from overseas".
"There is no light at the end of the tunnel," he said. "Do councillors want to go ahead with something when the system is already on its knees?
"We urge them to please talk to experts in the NHS and get their advice before they move on with this."
Lancaster MP Cat Smith said: “I have been in correspondence with Dr Jackson and Dr McClelland and I share their concerns, as well as wider concerns about the shortage of GPs we face nationally.
"It’s essential when planning large housing developments that consideration is given to expanding the number of services to meet the demands of an increasing population.
"I have raised my concerns and those of constituents with local councillors and I hope they will take these into account in their decision making.
"The city council’s plan for around 3,000 homes in south Lancaster includes provision and funding for additional health and education facilities and as such I hope these plans are approved.
"The alternative is for private developers to come in and they have no statutory obligation to provide services like GP surgeries.
"I’ll be following developments very closely.”