General Sir Richard Dannatt branded Mr Brown "malign" for failing to fund the armed forces adequately and said Mr Blair lacked the "moral courage" to make his then chancellor deliver the money that was needed.
The accusations come in a book, entitled Leading From the Front, serialised in the Sunday Telegraph today.
Gen Dannatt was unusually outspoken about the equipment and manpower needs of the Army during his time as Chief of General Staff from 2006-09, and some commentators believe he was passed over for the post of head of the armed forces as a result.
Shortly after his retirement from the military last year, it was announced that he would become a defence adviser to Conservative leader David Cameron, but he decided not to take up a post with the Prime Minister after he took office in May.
In his book, Gen Dannatt says that evidence for Iraq's possession of weapons of mass destruction - the official justification for Britain's involvement in the 2003 invasion - was "most uncompelling" and the planning for the aftermath of war an "abject failure".
While the 1998 Strategic Defence Review (SDR) provided a "good framework" for defence policy in the Labour years, it was "fatally flawed" by being underfunded by Mr Brown's Treasury and could not cope with the strains of deploying troops in both Iraq and Afghanistan at the same time.
Gen Dannatt reserved his fiercest fire for the two politicians at the head of the Labour administration.
"History will pass judgment on these foreign adventures in due course, but in my view Gordon Brown's malign intervention, when chancellor, on the SDR by refusing to fund what his own government had agreed, fatally flawed the entire process from the outset," he wrote.
"The seeds were sown for some of the impossible operational pressures to come."
Mr Blair "lacked the moral courage to impose his will on his own chancellor", said the general.
Speaking to the Sunday Telegraph, Gen Dannatt said: "I was the professional head of an organisation that was hurting, being asked to do an awful lot.
"I felt that the top end of the government did not understand or fully appreciate the pressures the Army was under and I tried really hard to get that understanding across.
"I felt it was pushing a rock up a steep hill pretty much all the way through. It was frustrating because from the land forces' point of view, we always do our job, but we knew we couldn't do it as well because we hadn't got the resources we needed."
Gen Dannatt said he felt that from Labour's arrival in office in 1997, Mr Blair was "prepared to use the armed forces on the world stage", but Mr Brown as chancellor was "not particularly interested in defence".
But he did not absolve Mr Blair of responsibility for funding shortfalls: "Every organisation has its tone set by its leadership.
"To me it seems extraordinary that the prime minister, the number one guy, cannot crack the whip sufficiently to his very close friend, the chancellor, and say 'We're doing this in the national interest, Gordon, you fund it'."
Margaret Thatcher's war cabinet at the time of the Falklands War in 1982 did not even include a representative of the Treasury, he pointed out.
Gen Dannatt complains that after Mr Brown became prime minister in 2007, he did not have regular enough opportunities to talk with him about the problems the Army was facing.
At one point in early 2009, he was forced to "ambush" Brown during a chance encounter in Horse Guards Parade to get across his concerns about Afghanistan, where British troops were suffering high casualty rates, he said.
Gen Dannatt did not challenge Mr Brown's evidence to the Chilcot Inquiry into the Iraq War, where the then prime minister said that generals were given all the equipment they asked for.
"Narrowly that's right," said the general. "Under operational requirements protocol, the Treasury is absolutely obliged to pay for the additional costs of operations. But we certainly weren't being given everything we asked for in the wider sense."
Gen Dannatt warned that Britain cannot ask its troops to stay in Afghanistan beyond 2014 or 2015 if casualty levels remain at their current rate.
"We've got to have cracked it by 2014, 2015," he said. "You couldn't ask an organisation to go on taking this level of casualties for 10 years."
Gen Dannatt also raised questions over Chancellor George Osborne's insistence that the estimated £20 billion cost of Trident renewal must come from Ministry of Defence budgets, rather than being paid for by the Treasury.
Warning that the "huge" cost of Trident will come on top of cuts to the MoD budget totalling as much as £35 billion over the next few years, Gen Dannatt said: "I don't know whether the full implications have really been understood."
Comments