The Royal National Lifeboat Institution is warning people not to be tempted to swim in the River Thames in London during the hot weather.
The advice is being issued as temperatures are set to rise to up to 34 Celcius in the capital on Saturday.
The temperature of the River Thames is just 10 degrees at the moment and the RNLI is warning that people can quickly get into difficulty if they end up in the water.
The Thames holds particular dangers because of its strong currents, high volume of river traffic and poor underwater visability.
Anyone who finds themselves struggling is advised to try and remain still, lie back and float.
There are RNLI lifeboat stations on the Thames at Teddington, Chiswick, Tower Bridge and Gravesend. Last year between them they rescued a total of 271 people from the river.
As the weather warms up lifeguards are also advising people from the London area who are heading to the coast to choose a lifeguarded beach.
Last year RNLI lifeguards aided over 30,000 people with incidents such as lost children, water sports incidents, people caught in rip currents, and also administering first aid.