Global Policy Forum

Lord Ashcroft, the Conservative donor and outgoing deputy party chairman, has been accused of avoiding more than £3m in tax by engaging in a financial manoeuvre the day before new legislation would have forced him to pay tax on all his income.

The BBC programme Panorama will report tonight that the peer, who steps down from his party role today, transferred the ownership of his main UK company, the Impellam Group, on 5 April. The 64-year-old peer transferred shares worth £17m in the company to a trust to benefit his children. The following day, a law came into force compelling all members of the Lords and Commons to be registered in the UK for tax purposes and pay tax on all their worldwide income. The law had been in large measure prompted by the controversy over his tax status.

Tax lawyer Richard Frimston is quoted telling the programme that Lord Ashcroft would have faced a large inheritance tax bill under the new legislation. Frimston said: "If that had been done on the following day, assets worth say £17m going into trust would have been subject to tax at 20%, which would have created an immediate inheritance tax charge of something in the region of £3.4m. So that was avoided by doing it on 5 April as opposed to waiting until 6 April."

Lawyers for the peer told the BBC: "Our client has denied any impropriety or wrongdoing in respect of any of the matters that you have raised."

Tonight, when asked about the tax avoidance allegation, a spokesman for Ashcroft said the BBC had made a fundamental error, but would not elaborate further. The peer is due to send a further response to the BBC tomorrow, he added.

In March, the peer, when revealing after a decade of speculation that he was domiciled abroad for tax purposes, said he supported the new law and expected to be "sitting in the House of Lords for many years to come". On the night of the general election in May he agreed with a BBC interviewer that he was going to become a "fully taxed person in Britain".

His admission of being a "non-dom" triggered a furore in the run-up to the general election. His close friend William Hague had nominated him for a peerage in 2000, on the apparent understanding that he would stop being a tax exile and return to the UK. Hague, then Tory leader, had said a peerage would "cost him [Ashcroft] and benefit the Treasury tens of millions of pounds a year in tax".

However it emerged this year that Ashcroft had not become a permanent resident when he took up his peerage. Instead he had persuaded officials to allow him to acquire a different status - as a long-term resident - meaning that the peer, who has a large business empire in the Caribbean and US, had not been paying tax on his income from abroad. Last week, Ashcroft told the Telegraph: "The negotiations [in 2000] with the government for me to join the House of Lords did not include any commitment on my part to be taxed on my worldwide income."

Tomorrow night's BBC programme says the transfer of his shares to his children's trust this year is legal and has not broken any rules. Panorama journalists have been investigating the peer for a year. Senior Conservatives are understood to have put pressure on Mark Thompson, the BBC director general and Sir Michael Lyons, the chairman of the BBC Trust, not to broadcast the programme in the lead-up to the general election.

Last week, Ashcroft, who has donated more than £11m to the Tories, published a report setting out his analysis of why the party failed to win a majority at the general election, saying it had not got its message and brand across to voters.

Panorama also says British detectives are engaged in a corruption inquiry in the Turks and Caicos islands, and are scrutinising loans to two senior politicians in the Caribbean country given by a bank controlled by the peer. One of the loans from the British Caribbean Bank, for $5m, was to the former premier Michael Misick who was accused by a judicial inquiry of appearing to become much richer after he won power. Britain last year took over the direct running of the former colony after widespread corruption allegations. Lord Ashcroft has said he was not engaged in corrupt dealings with Misick.