Temperatures in London and the southeastern part of the country reached their lowest levels in two months late Thursday as the British capital was struck by downpours after Tropical Storm Debby.

The mercury has fallen below 20 degrees Celsius for the first time since early June, the Met Office said.

In London, the temperature was 16 degrees Celsius as rain fell on Thursday afternoon, even cooler than recent overnight temperatures.

The southeast was hit by heavy rain on Thursday afternoon, sending the mercury plummeting.

Met Office meteorologist Becky Mitchell told the Evening Standard newspaper that Londoners can expect a brief break from the recent sweltering conditions.

Low pressure from Tropical Storm Debby, which was expected to dissipate over the North Atlantic late Thursday, sparked heavy rain in parts of the east and southeast on Thursday.

The Met Office said as much as 20 mm to 30 mm was expected to fall in an hour.

The average temperature will be 22 degrees Celsius. Britons started the week dealing with temperatures rocketing past 30 degrees Celsiu on Monday and Tuesday.

It came as scientists raised fears that "runaway" global warming could mean prolonged heatwaves become the norm in the UK.