According to Sky News, dozens of FTSE 100 companies are preparing to make warnings in corporate announcements over Britain potentially exiting the EU this summer.
The broadcaster said firms have begun discussing with auditors and lawyers the need to make such warnings later this year.
Firms in question include Barclays, HSBC and Legal & General as well as consumer staples Diageo and Unilever, the broadcaster reports.
The need to make such announcements would become more pressing if the Conservative party wins the general election in May, as the party has pledged to hold a referendum by the end of 2017.
They would mirror statements made last summer ahead of the Scottish referendum, when firms such as Royal Bank of Scotland and Standard Life warned of the risks to their business.
Separately, ratings agency Moody’s has stated Britain leaving the EU could cause damage to the UK’s credit rating, more so than any period of transition prompted by an uncertain outcome from next month’s general election.
This would be principally due to the negative implications on trade and investments with the continent, it said.