The “who planned it first?” row about inheritance tax rumbled on last night after the Treasury revealed that it was asked to produce final proposals in early September, three weeks before the Conservatives announced plans to raise the tax threshold to £1 million .

Downing Street insisted that the information proved that Labour was already planning the announcement, but the Conservatives said the records showed that Alistair Darling took an interest only after strong hints in August that they were about to prepare proposals.

The limited disclosure of details under the Freedom of Information Act proved that, as The Timesreported last week, Gordon Brown considered, but rejected at the last minute, plans to change inheritance tax. The information from the Treasury scotches the idea that the plan announced by Mr Darling in the Pre-Budget Report was cobbled together after the announcement by George Osborne at the Tory conference – but that was not the Conservative line last night and it does not prove that Mr Darling would have gone ahead.

Mr Brown omitted his inheritance tax plan on the Sunday before the March Budget. It is impossible to say whether Mr Darling would have done the same if Mr Osborne had not made his announcement on October 1. Whitehall sources told The Times that the assumption in the Treasury was that it would have been in the Pre-Budget Report and that detailed costings had been made.

The release of details came after Mr Brown told David Cameron in the Commons on Tuesday that the records would show that the Government was planning inheritance tax reform before the Tory conference.

The Treasury said its records showed that it first considered the proposal in January. Mr Darling confirmed that it was under discussion for the Pre-Budget Report on August 20. The Tories were swift to point out that that date was three days after a Tory policy group under John Redwood had proposed inheritance tax cuts.

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