It says that while London would suffer in a no-deal departure, it would fare better than the rest of the United Kingdom - nationally, the most damaging sort of Brexit would see economic output up to 3.3% lower in 2030 than would be the case with the status quo, which would widen inequalities.
Financial and professional services could be the hardest hit, with 119,000 fewer jobs nationally, the report claims.
The analysis by Cambridge Econometrics predicted a no-deal agreement could also cost up to half a million jobs, with as many as 87,000 disappearing from London alone.
Other sectors reported include science and technology (92,000 fewer jobs), construction (43,000 fewer jobs) and the creative sector (27,000 fewer jobs). The UK's economic output (measured by Gross Value Added) could fall by 3% by 2030 - the equivalent of £54.5bn.
Top British bankers asked Prime Minister Theresa May for more clarity about the future regulatory relations between the United Kingdom and the European Union (EU), although they were encouraged by the preliminary divorce agreement reached last month.
The economic forecast is based on five different Brexit scenarios, the worst of which is a "no-deal" exit that would lead to "lost decade" of an economic slump. It should help guide the government to the best outcome for London and the UK.
A Downing Street spokesperson said May provided an overview of the UK's position and update on Brexit negotiations.
"I've released these impact assessments because the British people and our businesses have a right to know the likely impact of the various options the government are considering on their lives and personal finances". A stand-off between Britain and the European Union over the future access to 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.
Khan said it was "astonishing" that Theresa May's government had failed to do any analyses. "Their complete lack of preparation is irresponsible leading to fears that they are putting party politics ahead of the national interest".
In a meeting with business leaders and chancellor Philip Hammond on Thursday, May promised she would prioritise formulating a new financial services trade deal with the European Union during the Brexit negotiations.
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