It would be "unconscionable" to drop below the Nato 2% spending target, the new chairman of the Defence Select Committee has said.
Tory MP Julian Lewis said he was "baffled" that David Cameron had not committed to continue meeting the target for spending 2% of GDP on defence.
He suggested that the "worsening situation" in terms of global security meant the UK should be looking to increase defence spending towards 3%.
He told Parliament's House magazine that defence had "fallen too low in the nation's scale of priorities".
Recent Nato figures have shown that the UK is one of just five of the organisation's 28 member states to meet the target of spending 2% of GDP on defence this year.
Money from a peacekeeping fund used to support fragile and war-torn states was included for the first time in the official measure of the UK's defence spending, helping keep it comfortably above the target.
Ministers have come under intense pressure to commit the UK to the 2% level for future years but Defence Secretary Michael Fallon has insisted that future plans will not be disclosed until Chancellor George Osborne's cross-government spending review in the autumn.
Mr Lewis said: " I'm baffled about it. The idea that we would drop below the minimum of 2% is just unconscionable as far as I'm concerned.
"We're facing both an international terrorist offensive and a more traditional threat from a potentially hostile state.
"Personally, in view of the worsening situation, I think we ought to be looking more towards a figure approaching 3%.
"Defence has fallen far too low in the nation's scale of priorities. The Government has got to justify the safeguarding and ring-fencing of budgets for other departments whilst leaving defence unprotected, and they have to justify this in the light of the constant and reiterated claim that defence is 'the first duty of government'.
"Well, if defence is the first duty of government why is its budget unprotected whilst the budgets of other departments are ring-fenced?"
The select committee chairman expressed concerns about the way the forthcoming strategic defence and security review will be conducted
"I think the next one will be done in exactly the same way as the last one was," he said, when the Government " appears to have said 'here is a certain sum of money, this is all you're getting, how much defence can you give me within that ... financial envelope'".
"But if you don't at least understand what it is you really want and need according to the actual and potential dangers which could and do face you, it is just an absurdity to say you are only going to get this sum of money irrespective, and you just have to make the best use of it that you can," he warned.
"You have to examine the threats, list the requirements and then you start arguing with the money men about how many you must have now, how many you can postpone without too much risk and how many you're unlikely to be able to get funded at all."