David Cameron has been given a sharp warning by MPs against any further cutbacks to Britain's armed forces in the wake of Russia's annexation of Crimea.
The Commons Defence Committee said the crisis in Ukraine underlined the continuing threat of state-on-state conflict - despite the ending of the Cold War.
It warned that Britain's national security depended upon its ability to maint ain a "credible deterrent" against a full range threats from cyber attack to a nuclear strike.
Further reductions in the size of the UK's conventional forces could ultimately call into question the effectiveness of the Trident nuclear deterrent, it said.
In a wide-ranging report, the committee expressed concern about recent remarks by former US defence secretary Robert Gates that cuts to defence spending meant the UK could no longer be a "full partner" to the US in the way that it had been in the past.
"If such concerns are being expressed about the degradation of the capabilities of the UK armed forces by our closest allies, these messages will not be lost on potential foes," it said.
Further cutbacks could leave the forces struggling to re-constitute military capabilities that had been axed, in the event of a new crisis emerging, the report warned.
It said that the Government's National Security Council needed to re-evaluate the threat level to the UK in the light of events in Ukraine when it updated the National Security Strategy next year.
It highlighted the way Russia's intervention in Crimea was accompanied by a test firing of an inter-continental ballistic missile, suggesting Moscow was prepared to use its nuclear capabilities "as a form of leverage in global relations".
"In a rapidly changing global environment, there is unlikely to be much warning of events that might require the reconstitution of conventional forces, once cut back, to adequately deter new and emerging threats," the report said.
"Recent events in Ukraine illustrate the speed with which new threats, and indeed the reappearance of old threats, can manifest themselves.
"The 2015 National Security Strategy must reflect that threats to UK security include the re-emergence of state threats that we may have been tempted to think had diminished with the end of the Cold War.
"These state threats may become manifest in a range of ways, including through attack with CBRN (chemical, biological, radiological, nuclear) weapons, conventional forces, terrorist proxies or cyber capabilities."
In order to be credible, the UK nuclear deterrent, had to be backed by sufficient forces to enable an escalation of conventional capabilities to the point where a further escalation to the deployment of nuclear weapons represented a "proportionate and appropriate response".
The deterrent strength of the armed forces against conventional military attack was, in turn, dependent upon their ability to project military "fighting power," the report said.
"This includes not only the physical capabilities of the armed forces, but also the conceptual and moral components reflecting a readiness to undertake operations," it said.
"There may come a point where further reduction in the size of the UK's conventional capabilities brings into question the effectiveness of the nuclear deterrent."
The committee's findings echo recent comments by the head of the Army, General Sir Peter Wall, that a sense of "moral disarmament" by the West following the conflicts in Iraq and Afghanistan was already affecting the calculations of potential adversaries and that a new war could come "sooner than we think".
For Labour, shadow defence secretary Vernon Coaker said: "The report is right to raise important concerns around the impact of Government defence policy on our allies' perceptions of UK capabilities.
"It is clear that this Government undertook their defence review with insufficient evidence."
Dr Andrew Murrison, Minister for International Security Strategy said: "Britain has some of the very finest and best equipped Armed Forces, underpinned by the world's fourth largest defence budget and backed up by the nuclear deterrent, the ultimate guarantor of our national security.
"With Nato and our allies, we will continue to play our part on the world stage and are ready to deal with threats, whatever they may be."
Defence Secretary Philip Hammond said Britain "may have to revisit" the understanding with the Russians that there would not be large-scale basing of troops from Nato countries on their borders and would keep "all our options open".
He told BBC Radio 4's Today programme: "We should be very clear that what we seek is a de-escalation of tension, a peaceful resolution of the crisis in the Crimea and ideally a return to a relationship with Russia where Russia accepts the norms of international behaviour and operates as a law-abiding state."
Mr Hammond said the public might have seen Britain's military action over the last two decades as being discretionary.
He said: "I think what this crisis will have done is reminded people that you don't always have that discretion, that you don't always set the agenda and the ultimate purpose of defence is to be able to defend yourself if you are attacked on an unprovoked basis by somebody else, and that's why we need to have a strong and resilient defence capability in the West and in the United Kingdom, and I will be arguing that we should maintain that as a vital part of Britain's defensive infrastructure.
"I'm comfortable with the budget we have got set out at the moment."
He added: "I wouldn't be comfortable if it was cut still further and I don't think some of our important allies would be comfortable if it was cut still further."