Labour and Tory MPs have renewed pressure on ministers to boost defence spending beyond the Government's current plans.
Vernon Coaker, a former shadow defence secretary, led the calls in the Commons by demanding that a forthcoming review should provide the resources needed to fight increasing threats facing Britain.
There has been widespread speculation about possible cuts amid major pressure on the defence budget.
"Our armed forces need resources that are over and above what is currently planned for them, particularly in light of the increasing threats we face as a country," said Labour MP Mr Coaker.
A strategic defence and security review was announced last year looking at all aspects of the UK's security capability, including defence.
But Mr Coaker said it was "completely wrong" for this review to start from a position of being "fiscally neutral".
He added: "How can you come to such a conclusion before all the strands of the review are finished?
"Surely this is about matching resources to threats, not the other way round.
"So let this be the line in the sand that ensures that this principle is at the heart of the decisions we take as we now move forward."
Mr Coaker was leading a backbench debate on defence, with a motion that defence expenditure should be maintained at least at current levels.
Tory backbencher James Gray (North Wiltshire) said this did not go far enough and that spending on defence "should be increased quite substantially".
Mr Coaker cited increased threats from Russia, China and North Korea, as well as terrorism and new technologies such as cyber attacks.
He urged ministers to stop dismissing media reports of defence cuts as speculation and involve MPs more in what the review was considering, such as providing more details about the costs of Britain's new F-35 warplanes.
Mr Coaker also challenged ministers to rule out further cuts to troop numbers in the Army, as well as cuts to the Royal Marines and axeing the amphibious assault ships HMS Albion and HMS Bulwark.