Defence Secretary Philip Hammond is to issue a sharp warning that there can be no going back on the cuts to the armed forces in the Government's strategic defence and security review (SDSR).
In his first major speech since taking over following the resignation of Liam Fox, he will voice his determination not to compromise current operations or constrain future defence capability.
At the same time, however, he will stress that only by tackling the "black hole" in the defence budget can the UK's military capabilities be sustained over the long term. "Unpicking the SDSR piece by piece is simply not an option," he will say.
Mr Hammond will also disclose that the final cost of operations in Libya had come in lower than previously estimated at £212 million - including £67 million for replacing spent munitions - all of which would be met from the Treasury reserve.
In his speech to the Royal United Services Institute in London, he will point to the Libya campaign as an example of the forces' ability to take on new commitments despite the cuts in the SDSR.
"This year, our armed forces have shown that - even with the enduring campaign in Afghanistan - they have the capability and the capacity to respond when the national interest requires," he is expected to say.
However he will say that after the defence budget "spiralled out of control" under the former Labour government, ministers now had to take action if the military was not to continue a "hand to mouth existence".
"I am clear that the purpose of the Ministry of Defence isn't simply to balance the books; it exists to defend the country," he is expected to say. "But the situation we face now - after the years of political failure to grip the problem - is that eliminating the black hole in the defence budget is the only way to sustain military capability over the long-term.
"If we don't reshape now we won't be in a position to order new equipment in the future. Our challenge is to move from the fantasy budgets of the past to firm foundations for the future. This is a transition that is essential to the future of defence - but no-one should be under any illusion that it will be easy or pain-free."
Mr Hammond will say that while he will neither "compromise current operations nor constrain future defence capability", there could be no going back on the cuts in the SDSR which have hit all three services.