The UK government has been warned it faces a "very substantial rebellion" from Tory MPs if it fails to find the money needed for the armed forces.
New British Defence Secretary Gavin Williamson was urged to ensure Chancellor Philip Hammond "digs deep in his pockets" and tells Theresa May about her duty to defend the country.
UK Conservative James Gray's warnings of a rebellion came after Mr Williamson sidestepped questions over accusations he did not make any formal representations to Mr Hammond for more defence money before last week's Budget.
Mr Williamson, making his debut at the despatch box in his new role, said he wanted to understand the threats faced by the UK, with a defence review ongoing across the British government.
The UK Labour party joined senior Tory MP Julian Lewis in putting fresh pressure on Mr Williamson about potential cuts at Defence questions.
UK shadow defence secretary Nia Griffith said Tory backbenchers were in "open rebellion" on the issue, with reports over the weekend that defence minister Tobias Ellwood could quit.
Mr Williamson told MPs he was yet to have a formal meeting with the Chancellor.
Speaking in the Commons, Mr Gray said of Mr Williamson: "His background ideally suits him to fighting the corner in the review that's coming along.
"So will he please 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 indeed the Secretary of State at the Cabinet Office, to remind them that they must not use this review as some kind of camouflage to cut our services?
"Will he speak to the Chancellor of the Exchequer to make sure he digs deep in his pockets to produce the money we need?
"And above all, will he speak to the Chief Whip to remind him if he does not do so he's going to be facing a very substantial rebellion?"
Mr Williamson replied: "I can assure him I will speak to every single one of those people that he has outlined.
"As he rightly points out, the defence of our nation is the first and primary responsibility of every government - that is one I take exceptionally seriously."
Mr Williamson added he would "do everything I can" to deliver for the armed forces.
UK Conservative former defence minister Mark Francois also told Mr Williamson: "The defence of the realm is the first duty of government above all others."
Mr Ellwood was earlier cheered by Labour MPs as he rose to answer a question about cadets.
UK Labour former minister Chris Bryant asked the minister to visit his Rhondda constituency, noting "he may have some spare time in the near future".
Mr Ellwood replied: "I'm not sure how useful I'd be if I had spare time in the future."
For Labour, Ms Griffith warned it was not possible to "do security on the cheap" and said it was "surreal" to hear suggestions that Mr Williamson had made no formal pre-Budget requests to the Chancellor for more money.
In response to whether he did make any representations, Mr Williamson replied: "What we have to do is to ensure that we understand what the needs are for our defence, our armed forces, going forward.
"She may wish to rush into things and actually just demand and demand and demand.
"What I want to do is to make sure that we have the arguments ready, understand the threats that this country faces and make sure that we deliver for our armed forces.
"That is what the focus is going to be. I have many conversations with the Chancellor and I'm looking forward to many, many more going forward."
Ms Griffith said she would "take that as a no", adding: "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.
"With his own backbenchers in open rebellion and one of his ministers threatening to quit over cuts, 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 little bit rich" for Labour to lecture the Tories on defence and the Government was committed to increasing defence spending
Mr Lewis, chairman of the UK Defence Select Committee, told Mr Williamson: "When he does speak to the Chancellor, will he take the opportunity of reminding him that in the Cold War years we spent 5% of GDP on defence.
"Now, we spend barely 2% of GDP on defence, and perhaps a target nearer to 3% of GDP on defence might prevent our armed forces being further hollowed out?"
Mr Williamson replied: "I've always seen 2% as a base as against a ceiling, and I will certainly be taking on board his thoughts and comments going forward."