Brexit is the last thing investment banks needed.

Friday's currency swoons and stock rout－triggered by UK voters' surprise decision to withdraw from the European Union－herald even harder times for securities firms already struggling to improve earnings. While some trading desks made money in the initial turmoil, continued market volatility in months ahead poses danger to trading profits. Companies that hire banks to advise on takeovers and raise money face years of uncertainty as Britain negotiates new international ties.

Analysts on both sides of the Atlantic cut earnings estimates for the biggest investment banks on the expectation that securities sales and major deals will be thwarted by economic and political uncertainty and currency swings. Fees from that business are likely to "tank," dropping more than 30 percent this year at European banks, Sanford C Bernstein analyst Chirantan Barua wrote.

Analysts at Citigroup Inc and JPMorgan Chase & Co estimated lower underwriting volumes in the UK and Europe.

"In light of such uncertainty, a lot of primary deals will be put on hold in equity and debt," said Joseph Dickerson, an analyst at Jefferies International Ltd in London. On the bright side, "the next week is going to be OK in terms of trading volumes."

Bank stocks plunged worldwide in anticipation of less profit as the British pound slid the most on record and global equities lost more than $2 trillion of value.

Hans Humes, who runs hedge fund firm Greylock Capital, a specialist in distressed bonds, said he watched fellow investors take a "step back", leading to wider spreads on relatively muted volume. Such caution threatens to stifle the corporate bond market.

Lower revenue in European markets will probably chop earnings over the coming four quarters by an average of 4 percent at the biggest US firms, Wells Fargo & Co's Matt Burnell said. Those banks and their European counterparts may face a risk-off approach from trading clients, may of whom could be dealing with their own issues of losses and redemptions. Trading units already were coming off their worst first quarter since 2009.