The prime minister has abandoned plans to impose a £17.5m cut to the training budget of the Territorial Army, saying it was the "right thing to do".
The U-turn came after Gordon Brown spoke to former defence secretary John Reid, amid calls from his own party to intervene and reverse the cutbacks.
Tory leader David Cameron said Mr Brown had been forced into a "humiliating climbdown" by the opposition.
On Monday, the government reduced the scale of the cuts from £20m to £17.5m.
It backtracked on plans to suspend all routine TA training for six months and offered a compromise of one night's training each month for personnel not due to be deployed to Afghanistan.
But Mr Brown told MPs that he had now decided that the TA training budget would remain untouched, meaning most units would continue to train one night a week, as well as one weekend a month.
The TA is made up of volunteers who commit their spare time to training as soldiers and working with the regular army.
It is divided into independent units who recruit locally and national units who recruit across the UK.
All TA soldiers who are mobilised have their civilian jobs legally protected
The issue has been raised at the last two prime minister's questions - including by Conservative leader David Cameron himself at last week's session.
At PMQs on Wednesday, Mr Cameron said he welcomed "the government's complete U-turn" on cutting the money from the TA, something he said had been brought about by questions from him, Conservative and Labour MPs.
But Mr Brown said the Tory leader was "wrong" to suggest he was not supporting defence as an extra £1bn had been spent on the Afghan mission and a further £1bn on defence.
"Having looked at all the issues ... I decided it was the right thing to do," Mr Brown said of the decision not to proceed with the TA cuts.
But the Conservative leader said he could not "even be straight forward when he's performing a U-turn" and the PM kept "getting it wrong".
In previous weeks Mr Brown argued those heading to Afghanistan would get the training required and said the cuts were because army chiefs wanted to focus on the regular Amy.
It should never have been on the agenda
Simon Hughes Lib Dems
But with a Conservative debate on the issue due on Wednesday afternoon, and nearly a dozen Labour MPs criticising the decision, Mr Brown changed his mind.
Former defence secretary John Reid and former defence aide Eric Joyce - who resigned last month over concerns over military policy - were among Labour backbenchers concerned about the possible impact of reduced training on recruitment and retention of territorials.
Labour MP Lindsay Hoyle had urged Mr Brown to intervene personally in the dispute, likening it to the situation earlier this year over the Gurkhas when the government was embarrassingly defeated in a Commons vote.
BBC political editor Nick Robinson said Labour MPs were willing to take Mr Brown on because his authority had been "gravely weakened" by his handling of the expenses scandal.
But Transport Secretary Lord Adonis said the prime minister should be applauded for acting on the concerns of MPs.
"When governments do take the right decision, why not give them credit for doing the right thing?" he told the World at One.
"The government has done the right thing by the TA and by those who are serving abroad...We have restored the funding. TA officers will get the full training they received before and I think that is the right thing."
For the Lib Dems, Simon Hughes said the decision to cut TA training in the first place was "completely bizarre".
"It should never have been on the agenda," he said.
"All the argument about our troops in Afghanistan and elsewhere is that they have not been given the basic kit and training."
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