The case must be made for a bigger defence budget to ensure Britain is able to respond to the changing threats facing the country, the vice chief of the defence staff has said.
In an interview with the Times, General Sir Gordon Messenger also warned that there are capabilities being produced by countries which could harm the UK.
The serving Royal Marines officer told the newspaper that "defence affordability is not something we should shy away from".
"We should be making the case for a bigger defence budget in order to respond to those types of threats that are changing all of the time," he added.
"What I am not suggesting is that we are about to descend into world war any moment now, but I do think there are activities going on that need to be countered."
Gen Messenger also warned how more cash was needed to match the expanding capabilities and weapons being developed by states such as Vladimir Putin's Russia.
When asked why the public should support calls for a bigger budget, he said: "There are capabilities being produced by those states that don't see the world in the way that we do, that could do us harm."
He also told the newspaper that Britain needs to be prepared for a "deterioration in the international arena" within 10 to 15 years.
The 55-year-old, who is tipped as one of those who could replace Air Chief Marshal Sir Stuart Peach as Britain's military head later this year, said the traditional lines between war and peace have been blurred.
Gen Messenger said this is down to certain states and terror groups who pose a threat to the rules-based system which has maintained global peace since the end of the Second World War.
"There is a duty for us, as a key influential nation... to have capabilities that counter it, deter against it and from a national perspective defend ourselves and our allies against it," he added.
His comments come amid at least a £20 billion black hole in the budget over the next decade and follow similar remarks made by head of the Army, Sir General Nick Carter in January.
In a speech at the Royal United Services Institute, Gen Carter warned how Russia has an "eye watering" capability that the UK would struggle to match without a bigger defence budget.
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.