A 14-year-old boy was missing on Monday evening and believed to have drowned while swimming in the River Thames, according to London’s police service, as thousands flocked to stretches of water to escape Britain’s scorching heat wave.

Late Monday afternoon, London’s Metropolitan Police was alerted to reports that a child had entered the river in Richmond, in southwest London. Members of all three emergency services and some civilians carried out an extensive search, but it was called off after they were unable to locate him.

“Despite the very best efforts of all involved, we must now sadly conclude that this young boy has died,” Superintendent Richard Smith, of the Metropolitan Police’s South West Command Unit, said in a statement. “Efforts to recover his body will continue into this evening.”

Authorities have warned of the dangers of swimming in the Thames and other bodies of open water in recent days, as people rushed to increasingly popular “wild-swimming” spots.

“I know that on days like today when temperatures are at a record high, it might look appealing to jump in and cool off in rivers, reservoirs, lakes or other open water,” said Superintendent Smith. “Please don’t.” The boy’s presumed death, he said, showed “the terrible consequences of what happens when it goes wrong.”