A "hard Brexit" could have a "crippling effect" on homebuilding in the capital, the mayor of London Sadiq Khan has warned.
One in four construction workers in London is from the European Union (EU), a report by City Hall has found.
The 'Housing in London' study found that 95,000 of the capital's 350,000 construction workers are from the EU.
The government has so far refused to confirm what the status of EU workers in the UK will be after Brexit.
Just over half of London's construction workforce are from the UK, while nationally overseas workers make up 13%, City Hall reported.
In the last two decades the population of London has grown by 25% but the number of homes has only increased 15%, it was claimed, placing huge pressure on the city's housing market.
Last week, a study by the campaign group Fifty Thousand Homes found a third of London homes granted planning permission had not been built.
The government has said it wants a million homes built across England by 2020.
London needs up to an extra 13,000 new construction workers each year until 2021 in order to plug skills gaps and meet the additional demands on the construction industry, according to City Hall.
Mr Khan said: "London is in the grip of a serious housing crisis."
"A 'hard Brexit' could leave a quarter of the skilled construction workforce in the capital high and dry which would have a crippling effect on our plans to build the homes Londoners so desperately need," the Labour mayor added.
GLA Conservatives spokesperson for housing, Andrew Boff, said: "London depends on migrants".
"We also need to train British workers to meet demand but we can't expect the industry to wait for us to catch up," he added.