Britain could lose almost 500,000 jobs and £50 billion investment over the next 12 years if it fails to agree a trade deal with the European Union, according a report commissioned by London Mayor Sadiq Khan.
Cambridge Econometrics, an economics consultancy, looked at five different Brexit scenarios, from the hardest to the softest form of Brexit.
It also broke down the economic impact on nine industries, from construction to finance.
The study said that in a no-deal scenario, the industry that fares the worst will be financial and professional services, with as many as 119,000 fewer jobs nationwide.
"If the government continue to mishandle the negotiations we could be heading for a lost decade of lower growth and lower employment," Khan said.
"Ministers are fast running out of time to turn the negotiations around," he added.
Britain and the EU will soon begin the much harder task of defining their future trading relationship, after settling the broad terms of their divorce settlement last month.
A stand-off between Britain and the EU over the future access to the single market for London's vast financial services industry is shaping up to be one of the key Brexit battlegrounds before Britain is due to leave the bloc in March 2019.