Gordon Brown is to clarify his evidence to the Iraq Inquiry after conceding that the defence budget has not risen in real terms every year under Labour, he told MPs today.
The Prime Minister told Sir John Chilcot's panel when he appeared before it earlier this month that the defence budget was "rising in real terms every year".
But later House of Commons figures showed this was not the case.
Asked at question time whether he would correct the record, Mr Brown said: "Yes. I am already writing to Sir John Chilcot about this issue.
"Because of operational fluctuations in the way the money is spent, expenditure has risen in cash terms every year, in real terms it is 12% higher, but I do accept that in one or two years defence expenditure did not rise in real terms."
Mr Brown made the assertion to the inquiry as he rejected accusations that as chancellor he starved the military of the funds it needed.
"The Iraqi expenditure was being met, but at the same time the defence budget was rising in real terms every year," he told the panel.
Repeating his claim, he told them: "The spending review of 2004 gave the Ministry of Defence a rising level of real spending, moving from 1.2% to 1.4% in real terms each year."
The Prime Minister's evidence sparked condemnation from some senior military figures.
The head of the armed forces at the time of the 2003 invasion accused him of being "disingenuous" in saying that he provided military chiefs with everything they asked for.
He conceded he had been wrong over the real-terms rise in the Commons under pressure from Tory MP Tony Baldry, who said House of Commons library figures showed the PM had been "simply incorrect".
Opposition leader David Cameron told him: "In three years of asking the Prime Minister questions, I do not think I have ever heard him make a correction or a retraction.
"The fact is, if you look at defence spending or defence budget cuts, there have been years with real-terms cuts and at last the Prime Minister has admitted it."
Downing Street said the Prime Minister would write to Sir John - who he referred to as Lord in the Commons before correcting himself - "shortly".
But a spokesman would not reveal when or how the PM became aware of the discrepancy in his evidence.
Mr Brown has rejected the criticisms of ex-military chiefs.
Two former Chiefs of Defence Staff - General Lord Guthrie and Admiral Lord Boyce argued that while urgent operational requirements were always funded, the Treasury failed to maintain the MoD's overall budget at a level needed to fight two wars.
But in an interview with BFBS, the forces broadcaster, Mr Brown insisted: "I think they are wrong. To be honest, I don't think it is appropriate for people to criticise us for not providing what we did provide.
"The urgent operational requirements that were asked for by our forces were always met."