United Utilities said the North West's supply of tap water - which comes from reservoirs in the Lake District - is less than half what it should be at this time of year.
It said levels should be around 70% in September, but reservoirs at Haweswater and Thirlmere are currently only 34% full (nearly 6% lower than last week). This time last year, the levels for the reservoirs stood at 75%.
An unusually dry summer in the Lake District is the reason for the worryingly low water levels, say United Utilities, which delivers around 1.8 billion litres of water a day to more than 3 million homes and businesses in the North West.
In fact, it says this summer - from June to September - has been the driest in the Lake District for more than 130 years.
A similar warning was issued last year when reservoir levels dropped due to warm weather and the Covid-19 lockdowns.
United Utilities is now contacting households urging them to take steps to save water.
In an email sent to customers on Wednesday (September 22), United Utilities said: "The water levels in the reservoirs which supply your water have continued to drop again this week. Please help us by doing all you can to save or re-use water now.
"The recent dry spell and warmer weather over the summer, along with more of us staying at home during the holidays, working from home or choosing to holiday more locally, has meant a big increase in the amount of water being used.
"We’re supplying 59 million more litres of water every day to meet demand. The impact of this has reduced levels in some of our key reservoirs across the region."
How you can help
It added: "To keep precious water in the reservoirs until they have a chance to recover and help protect local wildlife, we’re asking you to do all you can to cut down on the amount you use and save as much water as you can.
"We can do this with very little impact on our usual routine and every drop is precious, so swap a bath for a four minute shower, use the washing machine once less each week and re-use water where you can.
"Another simple way to make a big impact is to turn the tap off when you brush your teeth, which will save around 24 litres a day, enough for 96 cups of tea."
You can find more tips on how to save water here.