The mayor of London has activated emergency cold weather measures with temperatures forecast to drop as low as minus 6C overnight in the south and east of England.

The Severe Weather Emergency Protocol (SWEP) ensures that councils and charities across London open emergency accommodation for homeless people when temperatures drop below freezing.

Sadiq Khan praised “tireless” relief organisations for their efforts to prevent the homeless suffering in the conditions.

“Across the capital, we are doing everything we can to prevent anyone sleeping rough in these freezing conditions,” he said.

The Met Office expects the temperature to plunge throughout Wales and southern England overnight, with minus 6C temperatures forecast at Benson, Oxfordshire and Santon Downham, Suffolk.

It has warned commuters to beware of frost, fog and freezing fog on Tuesday morning and is considering whether to issue a weather warning overnight.

A spokesperson told the PA news agency: “There isn’t a warning out at the moment given there is a bit of uncertainty still with regards to the extent and density of fog patches.

“But there might be one issued overnight once we start to see things forming and when we have more of an idea about how things are panning out.

“For rush hour tomorrow morning, they could make for some difficult driving conditions in central and southern areas.”

Advice issued in conjunction with the RAC and National Highways tells drivers not to use full beam headlights because fog merely reflects the light, worsening visibility.

Motorists should also stay four seconds away from vehicles in front and wind down windows at junctions to listen for oncoming traffic if visibility is severely reduced.

Freezing fog could even lead to flight delays because planes will need to be de-iced before take-off, but the cold conditions are expected to ease by Wednesday.

The capital’s cold weather protocol has already been activated three times this winter and has helped 1,200 Londoners off the streets, an increase of more than 50%.

Mr Khan added: “London’s councils and charities will be working even harder this week to support some of the most vulnerable people in our city.

“On behalf of all Londoners I thank them for their tireless efforts.”

Data shows that 3,570 people slept on the capital’s streets between September and December last year.