US bank Citigroup would relocate almost 1,000 jobs outside of the UK if the government is unable to secure passporting for financial services.
The bank, which has 9,000 staff in the UK, has told MPs that failure to secure passporting would see it move the roles.
Citi has major locations in Dublin, Paris, Frankfurt and other European cities, meaning there would be no obvious destination for the roles.
Treasury Select Committee member Chris Philp revealed the sums in reference to a US bank in a hearing with chancellor Philip Hammond earlier today.
Although Philp did not name the institutions involved, City A.M. understands it was a direct reference to conversations with Citigroup.
A Citi spokeswoman declined to comment.
Only last week, Citi's UK boss James Bardrick warned that the main question facing the UK's financial services industry was when to enact contingency plans.
Addressing a conference in London, Bardrick said: “How do we and when do we start making decisions...knowing the plan is ready to go...it could be in the first quarter of 2017.”
It comes at a time when the City is worried that a so-called hard Brexit, which would preclude passporting, appears increasingly likely.
A number of big banks including HSBC and JP Morgan have previously warned losing passporting rights, which allow UK-based firms to carry out business in other EEA countries and vice versa, would leave them with no option but to shift staff outside of the UK, with some issuing warnings before the Leave vote was even announced.
Businesses are also worried that a transition period, which would extend the current set of rules for long enough for firms to get their house in order, will not be worked into the Brexit deal.
Indeed, Philp warned last month that a transition deal, including potentially a temporary Single Market membership, was necessary to stop banks activating their contingency plans before a Brexit deal is finalised.