Tony Blair has denied reports that ministers were instructed to ‘burn’ a report questioning the legality of the Iraq war less than three weeks before British forces invaded the country.

The Mail on Sunday quotes an unnamed senior No 10 figure saying that the then attorney general, Lord Goldsmith, presented a 13-page legal opinion on 7 March 2003 that suggested the war could be challenged under international law because of the lack of UN backing.

The paper’s source says: “There was pandemonium. The date when war was expected to start was already in the diary, and here was Goldsmith saying it could be challenged under international law. They said ‘burn it, destroy it’ and got to work on the [attorney general].”

A spokesperson for the former prime minister said that to Blair’s knowledge the reports were “nonsense”. “No one ever said that in his presence and in any event it would be quite absurd to think that anyone could destroy any such document,” they said.

“Mr Blair and Lord Goldsmith dealt with all the circumstances surrounding the advice at the [Iraq] inquiry at length and with all the documents. The fact is the advice given was that the action was legal and it was given for perfectly good reasons.”

On Thursday, Sir John Chilcot announced he would publish his long-awaited report into the Iraq war in June or July next year, giving government officials up to three months to carry out national security checks on its findings. In a letter to the prime minister, Chilcot said the text of his report – which is expected to be around 2 million words long – would be finalised in the week of 18 April 2016.

Blair’s office denied that he was the cause of the delay, saying he had replied to documents he received as part of the Maxwellisation process – in which witnesses who are to be criticised are given a right to reply – in August this year after receiving them in January.

Blair’s spokeswoman said: “Tony Blair has always wanted the inquiry to report as soon as it properly can and he looks forward to responding to the inquiry’s report. Mr Blair also wants to make it clear that the timetable of the inquiry and the length of time it will have taken to report is not the result either of issues over the correspondence between him as prime minister and President Bush, or due to the Maxwellisation process.”