The floods which have devastated central and western England in the past four days are the worst in modern times, the Environment Agency has confirmed - and the waters are still rising.

The swollen rivers Severn and Thames are not expected to reach their peak for a further 24 hours, although they have already put large swaths of town and countryside under water.

"We have not seen flood levels of this magnitude before," said Anthony Perry, an Environment Agency spokesman. "This is an extreme flood event. The 1947 event on the Severn has always been the benchmark, and this has exceeded it."

Further rain is forecast for the flood-affected areas on Wednesday and later in the week.

This morning, Gordon Brown, the Prime Minister, visited Gloucestershire, the worst hit county, as emergency workers were starting to evacuate 500 residents from the roads around Gloucester docks on the river Severn, where sewage was lapping into their homes on a dirty tide.

With the Severn already only 30cm from the top of the city's flood defences, the emergency services were predicting that the level of the slow-rising river would increase by up to a further 40cm in the early hours tomorrow.

Much of the town of Tewkesbury remained under water today after flash-flooding roared through the streets "like a train" at 10pm on Friday night. The RNLI used boats to ferry people to safety and RAF Sea King helicopters air-lifted people from their flooded homes.

Around 350,000 homes were without clean water supplies in Gloucestershire this afternoon, as supplies ran out to more and more homes in the county. The problems have been caused by the flash-flooding in Tewkesbury, where the Mythe water treatment works was overwhelmed.

Severn Trent Water was dispensing drinking water from 250 bowsers, or mini-tankers, and warned that it could be two weeks before water supplies were back to normal. The Army is on stand-by to start pumping out the Mythe treatment works as soon as the floods subside.

Long queues trailed outside supermarkets, as panicked residents tried to stockpile supplies of bottled water.

Forty-three thousand homes were without power after the flood-threatened Castlemead sub-station north of Gloucester was switched off, as 150 emergency workers battled through the night to keep the waters away from it. Eight high volume pumps today ringed the sub-station

Dr Timothy Brain, the Chief Constable of Gloucestershire, said that the key battle today was to prevent floods reaching the strategically vital Walham electricity switching station, which would plunge half a million homes into darkness.

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