Lloyd's of London, the centuries-old insurance market, has argued that Britain's continued EU membership would be the "better outcome" for the group in the looming June referendum.
Lloyd's, which made the comments in its annual report alongside news of tumbling 2015 profits, added that it was making contingency plans for a so-called Brexit.
Britons are set to vote in a crucial referendum on 23 June to decide whether to remain part of the European Union.
Pre-tax profits sank 30% to £2.1bn (€2.65bn) last year compared with £3bn (€3.8bn) in 2014, amid "challenging" conditions, the company added.
The performance was hit by low interest rates and low investment returns – which sank by £400m to £1bn last time around.
"It is already clear that the conditions we faced in 2015 are continuing into 2016," said Lloyd's chairman John Nelson.
"Uncertainty over the United Kingdom's membership of the European Union adds an unsettling ingredient to the mix.
"We believe strongly that continued membership would be the better outcome for Lloyd's and the businesses in our market but we are preparing contingency plans in the event of an exit."
Lloyd's added that it wanted to ensure continued access to the European Union, which is the world's largest insurance market.
"The global economic outlook remains challenging," the company said.
"Uncertainty over the UK's membership of the European Union could have an impact on Lloyd's access to European markets."