US Dept of Justice questioned RBS's head of forex trading. Reuters

The head of the Royal Bank of Scotland's European forex trading, James Pearson, has been secretly questioned by US Department of Justice (DoJ) in relation to alleged market rigging, according to sources cited by the Financial Times.

The US probe throws into question the Bank of England's (BoE) investigations into the matter. Pearson was previously questioned by the BoE but US prosecutors appear to be concerned about the thoroughness of that investigation, the newspaper reported.

A crunch meeting was held in April 2012 involving senior traders and BoE officials, including its chief currency dealer Martin Mallett, who is no longer with the bank. An agreement was reached whereby six banks would pay $4.3bn in fines.

Pearson was one of the select group of senior currency traders who regularly attended the meetings. He is not under investigation and is still employed at RBS.

The BoE commissioned an investigation and report on the matter which was carried out by leading commercial lawyer, Lord Anthony Grabiner QC. This investigation cleared the BoE and was described by it as "thorough and comprehensive".

Pearson is understood to have flagged up the meetings with the Financial Conduct Authority (FCA), stating that the BoE had been told about communication between traders relating to possible currency fixes.

The FCA, DoJ and BoE did not comment.