Chancellor George Osborne
The UK has lost its top AAA credit rating for the first time since 1978 on expectations that growth will "remain sluggish over the next few years".
The ratings agency Moody's became the first to cut the UK from its highest rating, to Aa1.
Moody's said the government's debt reduction programme faced significant "challenges" ahead.
Chancellor George Osborne said the decision was "a stark reminder of the debt problems facing our country".
"Far from weakening our resolve to deliver our economic recovery plan, this decision redoubles it," he added. "We will go on delivering the plan that has cut the deficit by a quarter."
But the BBC's political editor Nick Robinson said Osborne now risks being dubbed the "downgrade chancellor".
"Worse could follow if the Budget shows borrowing rising... but for most people, what will matter is not credit ratings or statistics but higher fuel, food and other prices and if interest rates go up," he added.
The UK has had a top AAA credit rating since 1978 from both Moody's and S&P.
Shadow chancellor Ed Balls said the decision was a "humiliating blow to a prime minister and chancellor who said keeping our AAA rating was the test of their economic and political credibility".
In announcing the ratings cut, Moody's cited the "challenges that subdued medium-term growth prospects pose to the government's fiscal consolidation programme, which will now extend well into the next parliament".
It added that the UK's huge debts were unlikely to reverse until 2016.
"The main driver underpinning Moody's decision to downgrade the UK's government bond rating to AA1 is the increasing clarity that, despite considerable structural economic strengths, the UK's economic growth will remain sluggish over the next few years due to the anticipated slow growth of the global economy and the drag on the UK economy," Moody's said.
But it added that the outlook for the UK is "stable", meaning it sees no further downgrades in the near future, and added "the UK's creditworthiness remains extremely high".