It now expects the "period of sluggish growth" to "extend into the second half of the decade".
Speaking on the BBC's Andrew Marr Show yesterday, Business Secretary Vince Cable dismissed the downgrade as "largely symbolic".
The Liberal Democrat said: "In terms of the real economy there is no reason why the downgrade should have any impact.
"If you remember last year the US was downgraded, the economy grew strongly relative to Europe... and France had a downgrade last year, its interest rates that it borrows long term in the markets are only a little above ours.
"These things do not necessarily affect the real economy but they reflect the fact that we are going through a very difficult time and we are trying to balance the need to get the deficit and the budget under control with the need to get back to economic growth."
He went on: "The rating agencies have a pretty bad record. They are a bit like tipsters. They get some things right and a lot of things not right.
Mr Cable also flatly ruled out deeper spending cuts, suggesting that kind of policy was coming from "right-wing ideologues".
He said: "I think to embark on a slash and burn policy in response to this would be utterly foolish and counterproductive, and I am sure we will not be going there."
Mr Cable said the argument about whether the coalition should shift from plan A to plan B was "a bit juvenile".
He added: "What we are actually talking about is plan A+, or plan A++. Of course you have got to have the budget discipline but you have also got to have the Government acting in a way that supports growth."
Tory former chancellor Ken Clarke warned it would take years to regain the top credit rating and return to "sensible economic growth".
But he said the coalition should "stick to" its policy, adding: "I think the way in which we will recover confidence is making clear we're a strong firm Government, that the strategy we're on is the one that is eventually going to get things better and that the alternatives frankly are a bit odd."
Former Labour chancellor Alistair Darling said he had been "extremely doubtful" of the Government's strategy ever since 2010.
He told Sky News' Murnaghan programme: "I think that when they were elected they very unwisely staked their reputation on maintaining the AAA credit rating that they had, they compared us to Greece, they said they could eradicate the structural deficit by 2015.
"These were wildly optimistic claims and they were perhaps made because of inexperience and maybe a touch of recklessness.
"But the result is that they have sustained quite substantial political damage, but more importantly for the country the economic harm of yet another another blow to confidence.
"I think that is very, very important, they have been following the wrong economic strategy, but they are paying a very, very heavy price for it."
