Former Defence Secretary Sir Michael Fallon has called on the government to spend more on Britain's Armed Forces.
At his first speech since resigning from the Cabinet, he made 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.
That's 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."
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.
He 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".