David Cameron has warned that defence cannot be spared further cuts in the forthcoming spending review.
The Prime Minister said the overall budget would be kept at £33 billion until the end of the parliament - insisting the coalition had to make "difficult decisions".
But he argued that the armed forces would still have some of the best equipment in the world at the end of the process.
Fraught negotiations have been taking place over the spending settlement for 2015-16, which is due to be unveiled in June. Defence Secretary Philip Hammond has cautioned that further curbs on his department could do lasting damage to military capability.
Mr Cameron said: "We simply had to make difficult decisions both on spending and on taxation. The decision we made on defence was not to cut the budget in cash terms but essentially to freeze it. The defence budget is around £33 billion and it is going to be that all the way through this parliament to 2015."
In a question and answer session on a visit to Glasgow, Mr Cameron pointed to the Government's existing commitment to increase equipment spending by 1% in real terms from 2015-16 onwards.
"But defence cannot be exempted altogether from difficult decisions," he added. "What I would say is look at what we are getting out of what we are putting in... Yes, we have had to make difficult decisions in terms of reducing the size of the army, the navy and the air force. But when you stop and think about the future equipment programmes that our services are going to have, I think we can be really proud of what we are getting in this country."
Earlier, Mr Cameron visited crew from the HMS Victorious, one of the Royal Navy's Vanguard-class submarines, in the west of Scotland.
Shadow armed forces minister Kevan Jones said: "There will be confusion and concern in the armed forces community. Past promises, like so many of our brave personnel, now seem redundant.
"Our armed forces deserve better than posturing from a Prime Minister who came to power promising a bigger army but has cut capabilities and manpower. Tough decisions are necessary in defence but the Government's plans are rushed and flawed."