Israeli online foreign exchange trading and spreadbetting company Plus500 has declared that some clients' accounts had been suspended as it sought to meet money-laundering rules.
As a result, shares in the firm shed over a third of their value, plummeting 36.3% in London Stock Exchange trading on Monday 18 May to end at 478p, its biggest daily fall, slashing its market cap from £860m ($1.3bn) to £540m in just a few hours.
London-headquartered Plus500 said on its website that the restrictions would remain in place until it completes a review of the information it holds on about 17,000 British customers.
London-based hedge fund Odey Asset Management holds almost 13% of Plus500 shares, according to Thomson Reuters data, while JP Morgan Asset Management holds a 6% stake.
Plus500 said it was also in "close talks" with the Financial Control Authority, the UK's financial watchdog, and assured shareholders that dividends would be paid in full.
"In addition, shareholders should note that Plus500's UK subsidiary, Plus500UK ... has in recent weeks been implementing certain enhanced client on-boarding and Anti-Money Laundering processes which have resulted in additional documentation checks being required," the company added.
"Until the review has been satisfactorily carried out, you will be unable to open any new trades on your account, deposit or withdraw funds," the statement said.
"If you have any open trades you will, however, still be able to freely service your existing positions with additional Maintenance Margin, although again, will not be able to withdraw funds until the review is complete."
Most of the company's operations are in Israel, and it sponsors Atletico Madrid football club.