The scandal engulfing the Royal Bank of Scotland reaches new heights today with serious allegations from a senior Labour politician that at least three of its former non-executive directors may have been intimidated and threatened with the sack for asking searching questions about its financial affairs.

The Observer can reveal that a former government minister, Lord Foulkes of Cumnock, who has been extensively briefed by former bank insiders, has written to the Financial Services Authority, the City watchdog, asking it to pursue the claims which, if true, could trigger a criminal investigation.

The intervention by Foulkes, who is also a member of the Scottish parliament and sits on the Commons security and intelligence committee, comes amid fears that the bank will be exposed as the UK's equivalent of Enron - the US trader that collapsed amid systemic fraud.

Last night Foulkes said there was "widespread public anger among the public and Parliament that bankers in the midst of this financial crisis appear to be profiting and no action is being taken in relation to action which could constitute criminal offences".

In relation to claims of intimidation, Foulkes said: "If it were to transpire that executives were pressured in such a way, then that is a most serious matter indeed that needs urgent action."

He is also understood to have been disturbed by claims that the bank misled investors over its exposure to bad debts. Yesterday it was reported that more than £30bn of "toxic" sub-prime mortgages were bought for RBS by traders in 2007 without the board being informed - a claim denied by the bank.

Foulkes's letter to the FSA chairman, Lord Turner, states: "You will be aware that there is widespread disquiet that, unlike in the USA, there appears to be no action being taken against any of the UK bankers who may have been culpable of one or more offences in their dealings."

He asks Turner to address "whether any knowingly false statements were made or prospectuses issued that could have led potential investors or depositors to believe the position was more favourable than the board knew it to be and whether there was any intimidation of non-executive directors who had been asking probing questions which led them to believe they would not be reappointed if they continued to pursue such searching questions". Matthew Oakeshott, the Liberal Democrat treasury spokesman in the Lords, said: "I have never come across such damaging claims of megalomania, cover-up and intimidation ... Never mind Northern Rock. I am really afraid that RBS will turn out to have been another Enron."

Foulkes's letter will be seen as the latest attempt by the establishment to up the ante on Sir Fred Goodwin, the bank's former chief executive, who has been blamed for its demise. Last month, Gordon Brown made a personal demand for Goodwin to hand back some of his £16m pension and pledged to take "all the legal action necessary" if he did not comply.

The bank's financial reports reveal that the former non-execs, who included Peter Sutherland, chairman of BP, Jim Currie, the former head of Customs and Excise and Steve Robson, a former adviser to the Treasury, were paid a basic fee of £72,500 a year. According to RBS, they were meant to "satisfy themselves on the integrity of financial information and that financial controls and systems of risk management are robust and defensible".

Concerns that the bank's non-executives failed to hold the board to account are bound to throw up further questions about how RBS was being run as it transformed itself from a relatively small outfit into one of the world's largest financial institutions.

RBS said the bank had not seen Foulkes's letter and could not comment. However, a source close to the bank said allegations that the non-executive directors were pressurised may have some foundation. "Bullied is too strong a word, but, like many companies, somebody is clearly the leader and they may throw their weight around," the source said.

Last month RBS recorded a loss of £28bn - the largest in UK corporate history. Its catastrophic collapse has forced the government to take a 75% stake in the bank. UK taxpayers are also having to underwrite billions of pounds of its toxic loans bought by subsidiaries in the US.

When contacted about Foulkes's letter, a spokesman for the FSA said: "We don't comment on whether or not we are going to pursue individual companies unless it is decided it is a matter of interest to the public."