The Bank of England (BoE) expects Britain to lose up to 75,000 financial services jobs after the country leaves the European Union in 2019, the BBC reports.
"I understand that senior figures at the Bank are using the number as a 'reasonable scenario', particularly if there is no specific UK-EU financial services deal," the BBC's economics editor Kamal Ahmed wrote, without providing a more specific source.
He added the BoE thought the figure could vary depending on the terms on which Britain left the EU, and that 75,000 was at the upper end of projections provided by other groups.
The BoE did not have an immediate comment on Tuesday.
Two BoE deputy governors whose roles cover financial services, Jon Cunliffe and Sam Woods, are due to speak to a parliament committee about Brexit on Wednesday.
The BoE is currently assessing British-based financial services firms' contingency plans to minimise disruption after Brexit.
Cabinet united behind May's Brexit vision
Britain says it's speeding up preparations for leaving the European Union, employing thousands more workers to make sure customs posts, laws and systems work on day one of Brexit.
Prime Minister Theresa May's spokesman, weakened after losing her governing Conservatives' majority at a June election, has been criticised by Brexit campaigners for being slow to prepare for a "no deal" scenario where Britain leaves the EU without an agreement on future ties.
With talks to unravel more than 40 years of union hamstrung over what Britain should pay to leave the bloc, Ms May is keen to nudge the talks forward to discuss future trading arrangements, aware that there are only 17 months of talks left.
"Alongside the negotiations in Brussels, it is crucial that we are putting our own domestic preparations in place so that we are ready at the point that we leave the EU," Ms May's spokesman told reporters on Tuesday.
"The preparatory work has seen a significant acceleration in recent months. Departments are preparing detailed delivery plans for each of the around 300 programmes under way across government."
The European Union's chief Brexit negotiator, Michel Barnier, said on Tuesday that he was ready to speed up negotiations with Great Britain.
Mr Barnier said that the agenda and dates for the next round of Brexit talks would be set "in next few hours or days".
Brexit campaigners are demanding that Britain leave with no deal if the talks do not move on beyond a discussion of the divorce settlement on so-called Brexit bill, EU citizens rights and the border with EU member Ireland by December.
But before leaving the EU, the government faces a struggle to get parliamentary support for a law to sever political, financial and legal ties with the bloc - the EU Withdrawal Bill against which lawmakers have tabled hundreds of amendments.