Suspicious trading occurred ahead of nearly a quarter of all merger and acquisition deals on the London stock market in 2005, according to a study released on Wednesday by the UK Financial Services Authority, which adds to global regulatory concerns that illegal insider trading is widespread.

US authorities do not keep statistics in the same way, but they have expressed similar fears about insider trading. The New York Stock Exchange referred twice as many suspicious trades to US regulators for possible investigation in 2006 as it did in 2004, and federal prosecutors have recently broken up two separate trading rings that allegedly capitalised on M&A information from Wall Street banks.