A no-deal hard Brexit could cost the UK half a million jobs and £50bn less investment, according to a study.
Research for the mayor of London Sadiq Khan warned of a "lost decade" of significantly lower growth.
The country could have 500,000 fewer jobs in the worst-case scenario and nearly £50bn less investment by 2030.
In London alone, there could be 87,000 fewer jobs and the capital's economic output could be 2% lower by 2030 than predicted under the status quo, it was warned.
The findings were in an analysis of the potential impact of five different Brexit outcomes on London and the UK, commissioned by the mayor last year from economic analysts Cambridge Econometrics.
Mr Khan said: "This independent analysis reveals the potential economic risks - and human costs - at stake in the negotiations.
"If the Government continue to mishandle the negotiations we could be heading for a lost decade of lower growth and lower employment.
"The analysis concludes that the harder the Brexit we end up with, the bigger the potential impact on jobs, growth and living standards.
"Ministers are fast running out of time to turn the negotiations around.
"A "no-deal" hard Brexit is still a very real risk - the worst possible scenario.
"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.
"This new analysis shows why the Government should now change its approach and negotiate a deal that enables us to remain in both the single market and the customs union.
"It's astonishing that the Government has failed to do any proper impact assessments on what Brexit could mean for our economy.
"Their complete lack of preparation is irresponsible, leading to fears that they are putting party politics ahead of the national interest."