The Thames Barrier has closed to protect 550,000 homes and properties from the risk of flooding .
A high tide forecast for Thursday (October 5) has led to the closure of the defence flood gates from 10am as a precaution, the Environment Agency said.
The move is to prevent the risk of flooding in London and along the River Thames catchment area as water levels rise.
Plans are in place for the gates to reopen at 4pm.
The Thames Barrier is one of the largest moveable flood barriers in the world, protecting 125 square km of central London.
It stretches 520m across the River Thames at Charlton and each gate takes 10 to 15 minutes to close - that's one-and-a-half hours for the barrier to completely shut down.
A control room is manned 24-hours-a-day, every day of the year.
Andrew Batchelor, Thames Barrier manager, said: “We are closing the barrier to protect London from the threat of flooding due to high water levels around the time of high tides from the sea."
Get advice on preparing for flooding or call Floodline on 0345 988 1188.
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