Some 350,000 homes in Gloucestershire will soon be without water because of flooding at a treatment works.
Tens of thousands of homes are without tap water and supplies in Gloucester, Cheltenham and Tewkesbury will run dry in hours, Severn Trent Water said.
About 600,000 people could lose electricity if flooding overwhelms defences around a key substation.
The Environment Secretary said contingency plans are being made to move some people out of the county.
The Chief Constable of Gloucestershire Police, Tim Brain, said the water supply was unlikely to be restored for a week, and that flooding has threatened to overwhelm an electricity substation.
"We have been focusing on this substation and the prognosis is that it will hold," he said.
Flood defences at the Walham Switching Station, which supplies homes in Gloucestershire and south Wales, were boosted overnight.
The Navy is also involved in protecting the Castle Meads substation, which supplies homes in and around Gloucester and has been turned off.
Mr Brain said this would be coming back on line on in stages from Monday evening, with power hopefully returning to parts of Gloucester soon afterwards.
About 43,000 homes in Gloucestershire have no power, with power cuts also taking one of the county's main sewage treatment centres off-line.
The Netheridge treatment centre stopped working when its electricity supply was interrupted.
Backup generators are on site and Severn Trent is stressing there is good capacity to store waste water at the site.
With regards to water supplies, the chief constable added: "With respect to the Mythe water treatment works that is still out of action and there I am afraid to say the prognosis is not good.
"The best case scenario is seven days and we have been told up to 14 days before it can become fully operative."
Bottled water and 600 water tanks have been brought in to bolster supplies, and shops in the area are reporting strong sales of basic supplies, with queues forming outside supermarkets.
Craig Lay, duty manager at one ASDA store, said: "We expected it was going to be busy, but when we looked out the doors prior to when we opened up at 8am there must have been... nearly 1,000 people outside waiting to come in. It looked like football crowds."
Police were called in to control crowds after one delivery of bottled water at the supermarket.
Fraser Pithie from Severn Trent Water said: "Most of Gloucester city has no water now; Cheltenham still has got water but obviously its likely to lose its supply later today."
Mr Brain said: "I must emphasise that the situation we face remains unprecedented.
"There are still high levels of water and there is still more water to come down the Severn and other tributary rivers and we have by no means passed the peak of the ongoing emergency. We are not yet in a full recovery stage."
Tewkesbury remains cut off, surrounded by water. Local Tory MP Laurence Robertson said: "The crucial thing at the moment is make sure everybody is safe."
But police are becoming increasingly concerned about a 19-year-old man reported missing in the town.
He was last seen in the early hours of Saturday leaving Montell's Night Club.
More than 100 Royal Navy personnel have been sent to the county to help the operation and Gordon Brown has also visited.
The prime minister said the response of the emergency services has been superb.
"We will provide crisis and care grants for people to make sure those who are displaced are given help and we will make funds available to local authorities in this area to make sure they can do what's necessary," he said.
Richard Hammond helped out with the rescue operation
The chairman of the Environment Agency, Sir John Harman, said the huge amounts of rainwater that had entered the river system were now heading downstream towards major centres.
He said the Severn would continue rising in Gloucester until Tuesday afternoon.
About 30 prisoners - whose cells were located on the ground floor - have so far been moved from Gloucester Prison to other jails as a result of the floods.
Around 80 more will be moved on Monday because of failing water supplies.
A number of public health issues have also been identified. The Director of Public Health for Gloucestershire Dr Shona Arora advised:
- Wear wellingtons to wade through the flood water and keep children from playing in it to minimise any risk of infection
- Boil all bowser water as a precaution and use it to make up infant formula
- Flush the toilet less often and try to use grey water including flood water for this
- Main hospitals are functioning, although Tewskebury Hospital has been partially evacuated
- Most GP surgeries are functioning
The Environment Agency has confirmed that the current floods are worse than those which hit the county in 1947.
BBC Radio Gloucestershire is running special programming with regular updates on the flooding.
It can be heard on 1413 MW, 104.7, 95.0, 95.8 FM, and online at bbc.co.uk/gloucestershire