A pledge by billionaire former Conservative treasurer Michael Ashcroft to live in the UK permanently and pay income tax as part of a deal to allow him a peerage cannot be verified, the parliamentary ombudsman has said.

Sir Michael Buckley was asked to investigate whether the controversial businessman, who gave millions to the Tories, had kept to his "gentleman's agreement" but his investigation was blocked by parliamentary rules.

It is essential Lord Ashcroft produces evidence that he did meet the Cabinet Office's requirements

Peter Bradley

Labour MP Peter Bradley requested the investigation after his own inquiries had been hampered.

In a letter published in the Guardian, Sir Michael told Mr Bradley that in terms of Lord Ashcroft's introduction into the House of Lords the "political honours scrutiny committee were content to rely on undertakings given by both him and the then leader of the Conservative party [William Hague]".

"Once Lord Ashcroft had been introduced into the House of Lords, however, the role of the committee ceased and their interest ended there.

"The Cabinet Office have also said that once a life peerage has been awarded it can only be withdrawn by an act of parliament. There would therefore have been no purpose in carrying out the kind of checks to which you refer."

Baron of Belize?

Lord Ashcroft was awarded his peerage, having been nominated by Mr Hague, only on condition that he based himself in the UK.

At the time he provoked further controversy after saying he wished to be known as Baron Ashcroft of Belize - base of his business affairs.

Mr Bradley wants Lord Ashcroft to lose the Tory whip

He later insisted the remark was a joke, but was also forced to give up his post as UN ambassador for Belize before taking up his peerage.

Wrekin MP Mr Bradley called for the withdrawal of the Tory whip from Lord Ashcroft and for any future donations from him to the party to be refused unless he proved he was living in the UK and paying British taxes.

"It is essential Lord Ashcroft produces evidence that he did meet the Cabinet Office's requirements before he took his seat and that he still fulfils them," said Mr Bradley.

"If he is unwilling or unable to do so, Iain Duncan Smith must take decisive action."

But a spokesman for Lord Ashcroft insisted that the peer had already complied with the agreement hammered out with the Cabinet Office.

Earlier this month Lord Ashcroft hit the headlines again after it emerged US investigators were looking at his links to Tyco - a Bermuda based group whose chief executive, Daniel Kozlowski, has been indicted on tax evasion charges.