Citigroup, Morgan Stanley and Bank of America are considering Ireland as an alternative location for some of their European activities if they need to move them out of the UK, according to people familiar with the banks, the Financial Times reported.
The FT reported that the plans were at a very early stage.
Bank of America and Morgan Stanley declined to comment on the article. Citigroup could not immediately be reached for comment outside regular working hours.
British Prime Minister David Cameron has vowed to conduct a referendum on a renegotiated EU membership if the Conservative party is re-elected in 2015.
This possibility has raised fears that the world's sixth-largest economy could quit the club it joined in 1973.
The situation worries many in the City of London, the financial centre that accounts for roughly one-tenth of the British economy.