Defence minister Tobias Ellwood is reportedly prepared to resign if cuts to the British Army take place. The minister for Defence People and Veterans is understood to have concerns about proposals which could see the Army's full-time strength reduced by 12,000 troops, to 70,000.
Mr Ellwood has allegedly shared his "deep discomfort" with colleagues about a list of cost-saving options faced by the Ministry of Defence, according to the Times newspaper.
A Whitehall source quoted by the newspaper said the Ministry of Defence was "beginning to try and push back" against the cuts.
The MoD said no decisions had been made and dismissed reports about the options being considered as "speculation".
Defence Secretary Gavin Williamson was said to be shocked by the "completely awful" headline proposals drawn up by military chiefs, according to a Times source.
An MoD spokesman said: "We have the biggest defence budget in Europe and are one of very few countries to not only meet but exceed NATO's 2% spending target.
"In the face of intensifying threats, we are contributing to the cross-government review of national security capabilities and looking at how we best spend the rising defence budget to protect our country.
"No decisions have been made and any discussion of the options is pure speculation."
Former Army officer and Defence Select Committee member Johnny Mercer MP said the cost of the Trident nuclear deterrent should not be part of the MoD's budget.
The Conservative MP said he was "advocating hard" for the cost of sustaining the continuous at-sea deterrent, which accounts for around 6% of the annual defence budget, to be borne centrally by the government.
"The whole thing needs more money," he said. "This is just a start - like the amphibious 'rebellion'. It's a campaign, not a silver bullet, to get defence back where it needs to be."
Liberal Democrat defence spokesman Lord Campbell of Pittenweem said: "When will the government listen and learn? Former members of all three armed services, informed commentators, members of both Houses of Parliament and now a defence minister have all voiced their deepening anxieties about possible heavy cuts in the defence budget.
"Reducing the Army even further, cutting the Royal Marines and selling surface ships will fracture our defence capability."