Britain's Environment Agency has issued severe flood warnings for parts of the River Thames.
Footage filmed from a helicopter on Tuesday showed the swollen river flowing through the capital London. The Thames Barrier was open at low tide to let water flow into the sea.
At high tide the barrier can be closed to protect central London from flooding.
Attention is now turning to areas further upstream, where some towns have been flooded and water levels are still rising.
Flooded homes along the river are being evacuated, but many more are at risk.
Parts of Britain have seen their wettest January on record and around 5,000 homes have been flooded, with some remaining under water for more than a month.
At first, the worst affected areas were in the south-west of England, but now more populated areas closer to London are also being hit. Many residents are angry about the slow response to the crisis and the lack of investment into preventing floods in the first place.
Last week, the government pledged an extra $236 million to help with the repairs and maintenance of flood defences, and the military have been drafted in to help build defences and evacuate homes.