Another US bank has threatened to cut its UK jobs if Britain votes to leave the EU in the referendum on 23 June.
Citi has told UK staff that it will probably look to "rebalance" its operations in the event of a Brexit. The bank employs 9,000 people across the UK.
"To continue to serve our clients and maintain efficient access to those markets currently enabled through the EU passporting regime, we would likely need to rebalance our operations across the EU," Citi's UK country officer James Bardrick wrote in a memo to staff.
The jobs warning follows a similar caution from JP Morgan boss Jamie Dimon last week.
"A vote to leave would be a terrible deal for the British economy," Dimon said.
"So if the UK leaves the EU, we may have no choice but to reorganise our business model here. Brexit could mean fewer JP Morgan jobs in the UK and more jobs in Europe."
JP Morgan employs 16,000 people in Britain.
Both Citi and JP Morgan are among the Wall Street banks that have donated to the Remain campaign.
Goldman Sachs and JP Morgan are understood to have pledged £500,000 each, and Morgan Stanley donated around £250,000. Meanwhile, Citi reportedly agreed to donate a sum significantly in excess of £100,000, and possibly as much as £250,000.
The UK is the biggest provider of staff for US banks in Europe - the five largest US banks employ 40,000 people in London, more than in the rest of Europe combined.
The tech boss said the company has invested over £1bn in creating new jobs in the UK's rail and energy sectors, but added: "Take away the UK’s membership of the EU, and the future investment case looks very different."