Cuts to the defence budget in recent years have left the British Army half as capable as it once was, a former Chief of the General Staff has said.
General Lord Richard Dannatt said there is a "very strong case" for Britain to increase defence spending, and that the military would benefit from an extra £5 billion a year.
Asked about the current state of Britain's military and whether it has been damaged beyond repair, he highlighted how the UK has some "very good equipment", but not enough of it.
General Lord Dannatt told the Press Association that when he was at the head of the Army in 2009 it was at a size of 102,000 but is now at a level of less than 80,000.
"What that represents is a situation whereby 10 years ago we could have five combat brigades rotating through a campaign in Iraq, and at the same time five combat brigades rotating through a campaign in Afghanistan," he said.
"We could not do that now. We could do one of those but we couldn't do both.
"The figures are actually quite harsh, for a 7% reduction of the defence budget which is what happened in 2010, it resulted in a 20% cut in the size of the Army, and I would actually suggest a 50% cut in what we can actually do."
His comments come ahead of the latest budget, and amid calls to increase spending to above 3% of GDP on top of a black hole in the budget of at least £20 billion over the next decade.
General Lord Dannatt, who was head of the Army between 2006 and 2009, highlighted how the UK is meeting the NATO target of spending 2% of GDP on defence. He said:
"But looking back in history we have never spent as little as 2%."
"During the Cold War years we were spending 5-5.5%, during the 1990s we were spending 4% and until quite recently it was 2.5-3%.
"So going down to 2% represents the smallest figure we have ever had, and it is buying us the smallest Navy, Army and Air Force we have had ever had.
"That is fine until we have another major conflict somewhere."
"Unless you have got a full range of capabilities, and you have got a crystal ball that is really accurate and can tell you exactly what the next threat is going to be - if you haven't got that full range of capabilities then you may have a gap in your response."
General Lord Dannatt said an increase in defence spending would send an important message to Britain's European partners, and would be welcomed by the Americans.
"And I think it would be a message not lost on Mr Putin or the Kremlin," he added.
With the autumn budget due to be set out by Chancellor Philip Hammond on October 29, General Lord Dannatt said Defence Secretary Gavin Williamson has made a good case for more cash.
He also stressed the Ministry of Defence, the Army, Royal Navy and Royal Air Force have a responsibility "to show that the money they are currently getting is being spent well".
"I think another £2-4 billion or £3-5 billion a year in addition to the £35-36 billion we are currently spending to get us to around about the 2.5% of GDP that would be right," he said.
An MOD spokesman said: "We have the largest defence budget in Europe, and it is increasing in real terms by £1 billion a year during this parliament.
"In October alone we have has seen over 2,700 personnel deploy to Norway to train alongside NATO allies and 5,500 troops arrive in Oman for the largest UK exercise in 17 years. This is a capable armed forces operating globally."