And the answer is half a million fewer jobs and almost £50bn less investment by 2030.
London Mayor Sadiq Khan, who commissioned the report, said if the government continued to "mishandle" negotiations, Britain could be heading for a "lost decade of lower growth and lower employment".
The professional and financial services areas would be worst hit with some 119,000 fewer jobs nationally, according to the study from the Mayor of London Sadiq Khan.
Brexiteers reminded the Labour politician of his responsibilities to serve London as its mayor after he released a damning study suggesting a Brexit no deal could see the United Kingdom economy lose £54billion by 2030.
Conducted by Cambridge Econometrics, the assessment also considers scenarios such as leaving the single market, the customs union - or leaving both.
"The report shows that a destructive hard Brexit could do much worse economic damage in regions like the East of England than in London", he said.
British Prime Minister Theresa May met financiers from firms including Goldman Sachs on Thursday to discuss the impact of Brexit on Europe's financial capital, as London's mayor said Britain could face a "lost decade" of low growth and investment.
Mr Khan is warning that with just 10 months to go, time is running out. "It is clear that a no-deal, hard Brexit without a transitional deal will place all this under threat".
"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", claimed Khan.
Other scenarios include continued membership of either the customs union or the single market, a transition deal phasing out membership, and a situation in which there is no transition deal, no membership of either the single market or customs union and no preferential trade agreement.
On top of this, he claimed London could struggle to retain a talented workforce in the art and entertainment sectors, and its creative institutions could stand to lose millions of pounds of European Union funding from streams such as Creative Europe, that would be hard to replace with government money.
The scenarios examined include leaving the EU single market, leaving the customs union, or failing to secure a transition deal by March 2019, described as a "hard Brexit" in the study.
A stand-off between Britain and the European Union over future access to the single market for London's 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.