George Osborne has insisted "we need to act now so we don't pay later" as he paved the way for fresh spending cuts in the upcoming Budget.
Savings equivalent to 50p in every £100 the Government spends need to be found by 2020, the Chancellor said.
But the cuts are "not a huge amount in the scheme of things", Mr Osborne told BBC One's Andrew Marr Show.
"My message in this Budget is that the world is a more uncertain place than at any time since the financial crisis and we need to act now so we don't pay later," he said.
"That's why I need to find additional savings equivalent to 50p in every £100 the Government spends by the end of the decade because we have got to live within our means to stay secure and that's the way we make Britain fit for the future."
Asked about claims the Government is facing an £18 billion blackhole in its finances, he replied: "£18 billion is the sum of money that has been revised off our nominal GDP. In other words, that's a number out there last year because inflation was lower.
"It's a real number in the sense that all around the world every Western country, and indeed in big emerging countries like China, Brazil, Russia, people are looking at economic prospects and thinking they are not as rosy as they were just a few months ago."