The BBC transmitted its Panorama programme into the financial affairs of Conservative donor, Lord Ashcroft, tonight – two weeks after pulling it from its schedules at the last moment.

But Panorama dropped one of the main allegations it had levelled against the peer – that he avoided a large sum of tax shortly before new legislation would have forced him to pay tax on his worldwide income.

The original programme had been delayed following objections by the peer hours before the broadcast. He had objected after the BBC had made the allegation in a press release issued to publicise the programme.

The allegation centred on a transfer of 25m shares in Impellam Group, one of the peer's firms. A spokesman for the peer had said the BBC journalists had made a fundamental error and misinterpreted a key company document released by Impellam Group.

In that announcement on 6 April this year, Impellam said "following a transfer of an indirect interest in the company, Lord Ashcroft no longer has a beneficial interest in 25,745,349 ordinary shares of 1p each in the company".

Tonight a spokesman for Lord Ashcroft said: "The BBC have realised what a cock-up they made the first time." But the BBC defended its decision to issue a press release before the programme was broadcast, saying the peer had not responded after two weeks to their questions about the share transfer. The BBC said its journalists were still looking at the transfer.

The BBC spokesman added: "Panorama has been carrying out this investigation for 12 months. It is detailed and forensic and for legal reasons has to be exact in its findings.

"It had been intended originally for the programme to go out earlier, but it was decided that it was not yet ready and more time gathering more evidence would make it stronger. It was a decision taken on purely editorial grounds and was in no way political."

The rest of the programme scrutinised his business activities in Belize and Turks and Caicos Islands where he has substantial commercial interests.

According to media reports, senior Tories urged senior BBC executives not to broadcast the programme before May's general election.

Ashcroft stepped down as deputy chairman of the party last month. He has been dogged by fierce political controversy over his tax status.

The businessman entered the House of Lords in 2000 after William Hague, then Tory leader, said the peerage would "cost him [Ashcroft] and benefit the Treasury tens of millions of pounds a year in tax".

In the lead-up to this year's general election, Hague was forced to admit that his statement was wrong. Ashcroft revealed after years of speculation that he was domiciled abroad for tax purposes.