Defence Secretary Gavin Williamson has been warned of a "substantial" revolt by Tory MPs in the event of more cuts to army numbers and naval capability.
Ex-minister James Gray urged ministers not to use a security review next year as "camouflage" for a cut in manpower.
Some Tory MPs have spoken out to warn the UK is at a defence crossroads.
In his first Commons outing since he took the job, Mr Williamson said he was "in awe" of British troops and would do everything he could to support them.
But Labour said Mr Williamson – who was a surprise choice to succeed Michael Fallon when he resigned last month – had shown little sign of trying to twist the chancellor's arm in the run-up to last week's Budget.
A mini defence and security review launched after June's election – intended as an update to 2015's strategic security and defence review in light of emerging threats – is expected to be published early next year.
Ahead of Monday's defence questions in the Commons, 25 Tory MPs signed a letter to Mr Williamson warning about the potential loss of amphibious assault ships HMS Albion and HMS Bulwark, saying it would "dangerously limit the array of options" available to the forces.
It follows unconfirmed reports that full-time army numbers could fall from their current level of about 78,000 to 70,000 – well below the government's target of 82,000.
Former Defence Minister Sir Nicholas Soames said the party had a "duty" to unite against further cuts to defence capability, while former soldier turned MP Johnny Mercer said a "campaign" was under way to "get defence back to where it should be" and the public did not want to UK's army to be "the size of Belgium's".
Mr Gray told the defence secretary that, as a former chief whip, he was well placed to "fight the corner" of defence in cabinet and urge chancellor Philip Hammond to "dig deep" to find the money needed.
"Will he speak to the prime minister and remind her that her primary duty is the defence of the realm?
"Will he speak to the national security adviser and the secretary of state for the cabinet office to remind them they must not use this review as some kind of camouflage to cut our services?
"Above all, will he speak to the chief whip to remind him if he does not do so, he is going to be facing a very substantial rebellion?"
In response, Mr Williamson said he regarded the UK's commitment to spend 2% of its income on defence "as a base, not a ceiling" and would lobby his ministerial colleagues on the MPs' behalf.
"When you see our armed forces and everything they do, and the commitment with which they give themselves to it, you cannot be anything other than awed by it. I will do everything I can to deliver for them."
Promising to reach his own judgement on the defence review, he said calls for Trident renewal funding to be taken out of the Ministry of Defence budget would be considered.
He added: "There has been a tendency since the 1990s to think the world is a much safer place than it actually is. The world is rapidly changing".
The defence secretary faced calls from Tory and Labour MPs to publish a specific defence industrial strategy following a consultation exercise earlier this year.
Shadow defence secretary Nia Griffith said Conservative MPs were in "open rebellion" over defence spending.
"You cannot do security on the cheap," she said. "We are hearing the marines may be cut by 15% and the Army reduced to 70,000, something which would seriously put our international credibility at risk.
"Just how bad do things have to get before the secretary of state does his job, stands up for defence and tells the prime minister and chancellor that enough is enough?"
Mr Williamson said it was a "bit rich" to be lectured by Labour on the issue of defence spending.