Downing Street has publicly refused to rule out cutting troop numbers as part of the defence and security review, despite the Defence Secretary insisting the Government would not "slash the size of the armed forces".
On Monday, the Prime Minister's spokesman declined to say whether a reduction in troops was being considered, stating only that ministers remained committed to increasing defence spending in line with the Conservative manifesto.
The manifesto does not commit to maintaining current Army manpower, although Boris Johnson has previously said the Government will "be maintaining the size of our armed forces".
Asked about reports over the weekend that defence chiefs had drawn up plans to slash the Army by up to quarter, the spokesman added: "It's just wrong to say that we plan to cut defence. We will fulfil our manifesto commitments, including to increase the budget above inflation."
He said the review, expected in the autumn, would be "underpinned by the commitments the Government has already made to continue to exceed the NATO target of spending two per cent of GDP on defence".
Government sources later said Downing Street was opposed to operating a smaller Army and was instead considering reducing the number of civilian staff in the Ministry of Defence.
The Telegraph also understands that Dominic Cummings, Mr Johnson's most senior adviser, offered reassurances on Monday that the review would be focused on "rebalancing" rather than an "overall reduction in numbers".
One insider added that rather than cuts, the review was more likely to lead to some military personnel being reassigned or retrained to reflect the changing nature and demands of the armed forces, including in cyber warfare and artificial intelligence.
However, asked whether he could "categorically" guarantee that troop numbers would not be cut, Ben Wallace, the Defence Secretary, appeared to go much further than Number 10 by insisting there was "no plan to slash the size of the armed forces".
Speaking in the Commons, Mr Wallace said "only a fool starts the debate on numbers rather than threat", adding that the review was not "driven by financial pressures".
"What is the case is our armed force should always, always be defined by both the threat we face as a nation, the capabilities we have and Britain's global ambition, and that is why, in the integrated review, we will deal with those processes rather than start the debate about numbers."
Commenting on a meeting held with defence chiefs last week, he added: "It was not a financial discussion, contrary to what was reported. It wasn't a numbers discussion either – it was a discussion about how we meet the threat and how we deliver a future armed forces to match that."
Mr Wallace also pointed out that both recruitment and retention within the armed forces had increased, telling MPs: "That is the direction of travel. It's very clear that our armed forces are growing, as is our commitment."
The Integrated Review, announced by Mr Johnson in February, will assess "Britain's place in the world", including its role in aid, development and counter-terrorism, in the biggest shake-up of its kind since the Cold War.
It will also look at how the UK can "better use technology and data to adjust to the changing nature of threats we face, from countering hostile state activity to strengthening our Armed Forces".
Mr Johnson has hired John Bew, a King’s College London historian, to conduct the review from Downing Street.