The Mayor of London has vowed to protect the Green Belt as he unveils his plans to alleviate the capital’s housing crisis by more than doubling the number of new homes built each year.

Sadiq Khan said his draft London Plan, which is due to be launched on Wednesday, will safeguard “the lungs of the capital”. He has also pledged to increase the city’s green cover to more than 50pc by 2050.

Developers have been nervously awaiting the strategy, after Mr Khan hinted last month that it would include a rule that 65pc of all new homes built in the capital must be affordable. After this suggestion was made, London house-builder Berkeley’s share price fell as investors took fright that the move would make many sites across the capital financially unviable.

City Hall sources suggested that the plan would instead call for a rate of 50pc affordable in developments across London, while private housebuilders would work towards a 35pc requirement. Last month, Mr Khan also said the target for the number of homes built every year will rise from 29,000 to 66,000.