Defence Secretary Philip Hammond has sparked a new outbreak of coalition feuding, calling for fresh welfare cuts in order to protect spending on the armed forces.
Tory MPs warmly welcomed Mr Hammond's declaration that he would resist further cuts to the defence budget in Chancellor George Osborne's forthcoming spending review.
But there was anger among Liberal Democrats, who are equally set on preventing a renewed squeeze on benefits. One Lib Dem MP hit back, accusing the Ministry of Defence of wasting money "gold plating" military equipment specifications.
The row erupted as Tory MPs smarting from the party's drubbing in the Eastleigh by-election demanded a return to "core" Conservative values. Backbencher Mark Pritchard said Mr Hammond could expect strong support from fellow Tories for his uncompromising stand on defence.
"Defence cuts have gone far enough," he said. "There will be zero political support from Conservative backbenchers for any additional cuts beyond those already announced in the Strategic Defence and Security Review. The Treasury needs to look elsewhere for additional savings."
However, Lib Dem MP Julian Huppert said building a fairer society meant helping those struggling on low incomes and Mr Hammond could not expect special treatment.
"He has to accept that in times of austerity when there are difficulties with spending money, it is important for us to be far more efficient in terms of Ministry of Defence spending, not to waste money on gold plating, but to provide the troops with the support they need and deserve, rather than to take money from people who really don't have spare money," he told BBC News.
In an interview with The Sun and The Daily Telegraph during a visit to the Royal Marines Arctic training camp in Norway, Mr Hammond said that a cut of just 0.5% in the welfare bill would be enough to provide the funding the military needs. "You take half a per cent out of the welfare budget, you've solved the problem in defence - half a per cent," he said.
Downing Street has already made clear defence will not be immune from cuts in the review of spending for 2015/16, expected later this year.
However, it is understood that it was comments by Lib Dem ministers Danny Alexander and Vince Cable calling for welfare to be protected and defence to be cut which prompted Mr Hammond to speak out. A Whitehall source said: "There is a real concern that the Lib Dems wants to protect the benefits culture at the expense of the armed forces."