British and U.S. regulators are poised to levy hefty fines on leading banks in a landmark settlement after a year-long global investigation of allegations of collusion and manipulation in the foreign exchange market.

At least one of the six banks set to be named early on Wednesday by Britain's Financial Conduct Authority (FCA) was still in 11th-hour negotiations over details of the deal, two sources close to the matter said.

The banks - UBS, Barclays , Royal Bank of Scotland, Citigroup, JPMorgan and HSBC - will only sign off on an expected combined penalty of 1.5 billion pounds ($2.4 billion) late on Tuesday, sources said.

It would be the first settlement over allegations of misconduct in the $5.3 trillion-a-day foreign exchange market.

The Wall Street Journal reported on Tuesday that Swiss financial regulator FINMA had sent warning letters to around 10 past and present UBS employees about potential enforcement action for alleged misconduct in the forex market.

A spokesman for FINMA declined to comment. Britain's FCA also declined to comment.

U.S. regulators and FINMA are also expected to be among those close to concluding investigations. Any U.S. settlement is expected to name at least one other bank, a source told Reuters last week.

Bank of America has said it was in "advanced discussions" with U.S. bank regulators.

The U.S. Commodity Futures Trading Commission was likely to allege false reporting and manipulation, lawyers said, the two most applicable options under U.S. law, which it also used in its settlement over Libor benchmark rates.

This is in contrast to UK authorities, which can look at charges such as failure to put in place the right controls. The two U.S. banking regulators are likely to go after the banks for failing to prevent bad conduct.

The U.S. Office of the Comptroller of the Currency is likely to settle charges with Bank of America, Citi and JPMorgan.

The Federal Reserve will be responsible for any settlement with the units of the four foreign banks. But it was not clear whether the Fed would announce its deal at the same time.