Britain will put its status as a "credible military power" at risk unless it increases its defence budget, the former commander of the UK's Maritime Forces has said.
Rear Admiral Alex Burton raised concerns over the country's ability to "fight and win on the front line" if more spending is not announced.
His comments come amid a black hole of at least £20 billion in the defence budget over the next decade, with widespread speculation about possible cuts to personnel and equipment.
Rear Admiral Burton told the BBC: "If you do not spend more on defence than we currently are as a percentage of GDP, then we put at risk the fact that we are currently a credible military power, and from that we put at risk our position on the global stage."
He added: "What worries me and worried me when I left the front line and was operating in headquarters is that some of the decisions we were making - and potentially over the next 12 months some of the decisions that will be made - will affect the ability to fight and win on the front line.
"And the challenge is ensuring that we're still able to do that (fight and win) not just over the next 12 to 18 months, but that we're able to do that over the next 10 to 20 years."
He also added that military threats needed to be taken seriously, and that without stronger military capabilities, Britain's "ability to stand up for our beliefs and protect our interests will be weakened".
Earlier this month, General Sir Gordon Messenger, the vice chief of the defence staff, said the case should be made for a bigger defence budget to ensure Britain could respond to changing threats.
The serving Royal Marines officer told The Times that "defence affordability is not something we should shy away from".
In January, the head of the Army General Sir Nick Carter warned in a speech that Russia had an "eye-watering" capability that the UK would struggle to match without a bigger defence budget.
The Treasury told the BBC that defence spending was already growing to meet "ever-changing threats" and that it would be increasing by £1 billion a year over the next three years.