Former defence secretary Sir Michael Fallon has called on the Government to spend more on Britain's armed forces as he made his first speech since resigning from the Cabinet.
Making the case for £1 billion to be injected into Ministry of Defence accounts this year, he said increasing GDP spending on defence to 2.5% would give the military an extra £7.7 billion annually.
His comments at the Defence and Security Forum on Monday came after a £20 billion black hole in the budget for the next decade emerged.
In recent months there has been widespread speculation about possible cuts to personnel and equipment owing to major pressure on the defence budget, alongside calls from MPs to increase spending to 3% of GDP.
Warning that Britain's security is at stake without an increased defence budget, Sir Michael said: "If we're happy to retreat from our vision of a confident, outward-looking Global Britain standing up for our people, our values, our allies, then we will drift downwards to being a bit-part world player, a part-time champion of democracy and freedom.
"That would mean walking away from our international obligations, letting down our allies, and in the end leaving us less safe.
"On the contrary, we should be doing more in the world: our troops, planes and ships should be seen on every continent, in every sky, on all seven seas. And that ambition needs a fully funded budget to match.
"The deficit is coming down. We are increasing spending in other priority areas, like the NHS and schools. So let's release an extra £1 billion to fire up the defence budget this year, and set 2.5% of GDP as our new target for the end of the parliament."
The Sevenoaks MP announced his resignation from the Cabinet in November amid allegations of inappropriate behaviour towards female journalists.
Admitting his actions had "fallen below the high standards required", he was one of the most senior ministers caught in a wave of allegations of improper behaviour around Westminster.
Sir Michael's speech marked one of the first times he has spoken with more freedom about the financial affairs of the MoD since his departure from the front bench.
Late last year he pledged to MPs in the House of Commons he would "find an early opportunity to speak out on the right level of defence spending to meet the threats that our country faces, and to do so more freely than the constraints of government allowed".
Speaking at the event in London, the 65-year-old said defence is the "first duty of government", and the "military can always be more efficient and they should be".
"Radical ideas like pre-positioning warships in the eastern Mediterranean and the Gulf, and doing more training with allies closer to home, need following through," he added.
"But in the end, defence needs a bigger budget because the threats are real and growing: they are at our borders, across our waters, on our streets and in our homes."
Sir Michael said the threats to Britain have "intensified", and that a security review being carried out by the Prime Minister's national security adviser "must recognise that".
Mirroring concerns voiced by chief of the general staff, General Sir Nick Carter, during a speech at the Royal United Services Institute on Monday, Sir Michael said Vladimir Putin's "intent to subvert western democracies" is evident.
Pointing to the fact the Kremlin is spending more than 5% of GDP on defence, he said Russia is investing in "conventional and nuclear forces, in hybrid and electronic warfare".
Sir Michael said the Nato 2% target of GDP spending is a "minimum", and added: "Increased threats must mean a bigger defence budget. Our security is at stake."