UK heatwave warning: Don't jump in Thames to cool off, London police warn as Met Office predicts hottest April day in almost 70 years

Londoners have been warmed not to swim in the Thames despite soaring temperatures as forecasters predicted today could be the hottest April day since 1949.

Although it might seem like a natural place to try and cool off, it is extremely dangerous, emergency services said.

"As tempting as it is to go for a swim in this weather (even for our dogs), remember the water is still very cold and dangerous," the Met Police's marine policing unit tweeted.

The London Ambulance Service meanwhile warned Londoners to stay hydrated and stick to the shade.

A 2012 byelaw bans people from swimming in the busiest section of the River Thames, between Putney Bridge and Crossness, unless they get advance permission from the Port of London Authority (PLA).

It is not illegal to do it elsewhere but the RNLI, PLA and Maritime and Coastguard Agency have all warned against it in the past.

"A boat having to stop suddenly or swerve to avoid a swimmer could put the boat and/or its passengers at risk of injury," says advice written by the PLA.

The by-law was brought in after David Walliams popularised "wild swimming" with a 140-mile Sport Relief swim.

It is considered highly hazardous due to strong currents, tides and whirlpools, while cold water shock can cause muscle weakness and disrupt heart rhythm.