The RBoS Shareholders Action Group, plans to sue RBS and its former chief executive Fred Goodwin, as well as former chairman, Tom McKillop, and the former investment bank chief Johnny Cameron for allegedly misleading investors at the time of a landmark £12 billion rights issue in 2008.

The shareholders allege that they were misled by omissions in the prospectus which concealed the frailty of the bank, which was bailed out by the British government during the 2008 financial crisis.

A few days ago, it was reported that RBS is being investigated for possible breaches of US sanctions against Iran. The Federal Reserve and Department of Justice are carrying out the probe. The investigation comes after RBS, parent firm to private bank Coutts, volunteered information to US and UK regulators 18 months ago, reports said. The US authorities are examining a number of banks for possible sanctions violations.

Barclays

Meanwhile, Varley, former Barclays CEO, is at the centre of a regulatory inquiry into unorthodox payments to Qatar's sovereign wealth fund, according to a report in the Sunday Telegraph newspaper.

The report said Varley was one of the four current or former executives whom the Financial Services Authority, the UK financial regulator, is investigating over "advisory fees" paid by the bank to Qatar Holding in 2008.

The payments were linked to the sovereign wealth fund's participation in an £11 billion refinancing of the UK bank at the height of the financial crisis.

Reports said the FSA and Barclays declined to comment.

Barclays confirmed last week the launch of a criminal probe into payments between Barclays and Qatar Holding. It said on Wednesday that the UK's Serious Fraud Office (SFO) had started an investigation into "payments under certain commercial agreements" between it and Qatar.