Restructuring the Army is "one hell of a risk" that will weaken the armed forces, one of Britain's most senior generals has warned.
General Sir Richard Shirreff warned that the "jury is out still" on plans to slash numbers in the regular army and substitute them with reservists, saying if the idea is going to work "the nation needs to get behind" it.
The general said defence cuts had "hollowed out" the armed forces, particularly the Royal Navy, which have been "cut to the bone" and left unable to take part in Nato maritime operations.
Russia's takeover of Crimea meant it was imperative for the UK to protect its defence budget, he said, even if that meant other departments suffered.
The general, the army's third most senior officer, stepped down from his post as Nato deputy supreme commander on Friday and will leave the Army in August.
His warning, in an interview with the Sunday Times, comes days after MPs warned Prime Minister David Cameron against any further cuts to Britain's armed forces in the wake of Russian's annexation of Crimea.
The Government is cutting the regular Army from 102,000 to 82,000 by 2020, while the newly-renamed Army Reserve - formerly the Territorial Army - is being expanded from 19,000 to 30,000.
General Shirreff said those implementing the changes had "made a pretty good fist of a very difficult hand of cards", but added: "I wouldn't want to let anybody think that I think that Army 2020 is good news, it's not.
"The sort of defence cuts we have seen... have really hollowed out the British armed forces and I think that people need to sit up and recognise that."
He told the newspaper his biggest concern was the impact of cuts on the Navy, which have left it without an operational aircraft carrier until 2020 and a fleet of just 19 frigates and destroyers.
The general said the Royal Navy's ability to participate in Nato naval maritime operations have been compromised, which affected how people think about the UK.
He said: "A hollowed-out navy means you can't project power. I've heard this said in the Ministry of Defence: 'The yardstick by which we measure ourselves is our ability to punch above our weight'. You can't do that now. By that yardstick, therefore, we're failing."
General Shirreff warned the question of whether the Army being more dependent on reserves would work or not was still unanswered, but he said it was "one hell of a risk".
He said it would need a "complete shift in culture" and support from the wider public and employers if it is to succeed, saying: "...the nation needs to get behind this. It's not just the armed forces - this is everybody's business."
The general, who has been co-ordinating Nato's response to the crisis in Crime and Ukraine, warned of further aggression by Russian president Vladimir Putin, the Sunday Times said.
Following Russia's "armed illegal aggression", the country has now become a "strategic adversary" of Nato, rather than a "strategic partner", General Shirreff said, and he argued that the UK and other European nations now need to protect their defence budgets to deter Russia, meaning cuts to other Whitehall departments.
He said: "We all support the efforts to get the deficit down but it is all about priorities. What really matters? Well, the first duty of government is to protect the nation. Defence is really, really important. And the electorate need to understand there is no point in having hospitals and schools and welfare unless the country is safe."
The Commons Defence Committee this week said the crisis in Ukraine underlined the continuing threat of state-on-state conflict, warning that Britain's national security depended upon its ability to maintain 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.
A Ministry of Defence spokeswoman said: "The UK maintains some of the very finest and best equipped armed forces, underpinned by the world's fourth largest defence budget.
"With a restructured, more flexible and agile Army and with £160 billion planned on new equipment over the next decade we will ensure our armed forces retain their formidable range of cutting-edge capabilities and ability to project power across the globe, including by maintaining naval patrols in the Gulf, counter-piracy operations and a presence in the South Atlantic in addition to our nuclear deterrent.
"The Royal Navy is now operating the world's most capable air defence destroyers, the most advanced attack submarines and building its largest ever ships, the new aircraft carriers."
Defence Secretary Philip Hammond said he was listening to critics of the Government's policies but "much of what I'm hearing is nonsense".
He told the BBC's Andrew Marr Show the UK still had the world's fourth largest defence budget and was a "credible, capable and reliable" ally to the US.
"Of course we have had to make savings, of course we have had to take some very tough decisions," he said.
"But we are working with the military chiefs to make sure we prioritise our very large defence budget, invest it in the areas that are going to matter in the future and I recognise that sometimes that has meant we have had to take decisions that have upset some people about legacy capabilities.
"But we are looking to the future, not the past."
He said while he would "like to have more assets", the Royal Navy still took part in Nato exercises, but was also involved in drug-busting operations, anti-piracy missions and humanitarian and other work, including assisting in the search for missing Flight MH370.
"The Royal Navy is busy around the world and, of course, we would like to have more assets but we are very effective at using the assets we have got to deliver the effect we need to do."
Shadow defence secretary Vernon Coaker said: " While I agree with the rationale behind the Army 2020 proposals, serious questions remain about their deliverability.
"General Shirreff's comments signal yet another senior military voice adding to the chorus of concern over the Government's defence reforms.
"The Defence Secretary has been too quick to dismiss these concerns and those of our allies. He has pursued his policy with little regard for the implications of its practical application.
"That is why Labour has urged the Defence Secretary to pause his reforms until it is clear that the Army 2020 restructure will not leave Britain with a dangerous gap in our military capability."