A UK Government minister in charge of stamping out corporate tax avoidance was part-time chairman of an Bermuda-based company which avoided more than £100m a year in taxes, according to a report in The Sunday Times.

The UK-based newspaper revealed that Lord Myners, who is already under fire for approving Sir Fred Goodwin's massive pension from Royal Bank of Scotland (RBS), was on the board at Aspen Insurance Holdings, just as Prime Minister Gordon Brown is seeking to win the backing of heads of government to prise open tax havens at a meeting of the G20 in London on April 2.

Lord Myners, who earned almost £200,000 from Aspen in one year, is also facing questions over share options he accrued during five years as chairman of the company, said the paper, with accounts for the re/insurer showing that he held 318,338 share options at the end of 2007. On Friday the shares, which are listed on the New York Stock Exchange, closed at $21.64, which would value that stake at £4.8m.

The financial services secretary to the UK Treasury and former chairman of the Guardian Media Group, was awarded the majority of the share options at an exercise price of $16.20 a share in August 2003, a year after he helped to start the company, the paper claimed.

When Lord Myners stepped down in May 2007, he was given until August 2008 to buy the shares and if he had bought them between January and August last year, when Aspen's share price fluctuated between $22.59 and $29.90, he would have made a profit of between £1m and £2.1m.

The Sunday Times, who approached Lord Myners on Friday morning, said he declined to comment whether he had exercised the shares, but on Saturday afternoon a Treasury spokesman said that he had not exercised the share options and owned no stock in Aspen.

Although any share transactions made by Myners while he was a director would have had to have been declared, these rules cease to apply once a director leaves the board.

A filing with the US Securities and Exchange Commission discloses that Myners, who is also leading the government's clampdown on City bonuses, received a farewell bonus of £50,000 for his final four months at Aspen.

His five-year term as chairman of both the UK and Bermuda arms of Aspen ended in May 2007 and he became a minister last October, according to the paper.

The US Congress recently named Bermuda as one of more than 30 "offshore secrecy jurisdictions" in legislation to crack down on havens in the coming weeks, while Brown has repeatedly called for tax havens to be outlawed.