God save the AAA rating.
The U.K. was knocked down one notch to Aa1, with its ratings outlook at stable. Moody's said the key drivers of the downgrade included the country's rising debt burden and tepid growth outlook over the next few years.
"[A]lthough the U.K.'s debt-servicing capacity remains very strong and very capable of withstanding further adverse economic and financial shocks, it does not at present possess the extraordinary resilience common to other AAA-rated issuers," Moody's said.
The U.K. had held AAA status since Moody's first began rating the country in 1978.
In December, the U.K.'s budget monitor projected that the country's economy would grow by just 1.3% this year. The government has been pushing a much-criticized austerity program, and finance minister George Osbourne said he remained committed to those efforts, even after the downgrade.
"This is a stark reminder of the debt problems that Britain faces and the clearest possible warning to anyone who thinks we can run away from dealing with those problems," he said. "Far from weakening our resolve to deal with our debts, this should redouble our resolve."
The British government has said its belt-tightening will have to continue until 2018.
In announcing the downgrade, Moody's said it expects the U.K.'s debt to peak at 96% of GDP in 2016, up from around 90% today.
A year ago, Moody's switched the outlook on the U.K.'s AAA rating to negative, in a prelude to Friday's downgrade. At the same time, the firm cut the ratings of half a dozen European countries.
The other major rating agencies, Fitch and Standard & Poor's, still have the U.K. rated AAA, though with negative outlooks.
The United States maintains its AAA rating from Moody's and Fitch, though it was downgraded by S&P in August 2011 following the debt ceiling standoff in Washington.
Steven Englander, a foreign exchange strategist with Citigroup (Fortune 500), said in a research note following the downgrade that the move was unlikely to raise borrowing costs for the U.K., as bond yields in the United States, France and Japan had remained stable following similar downgrades. But it increases pressure on the country to pursue growth by weakening the pound, he added. ,
"[W]hile by itself the announcement merely accelerates what was expected to happen at some point, the need for weakness [in the British pound] will become more apparent to policymakers and investors," Englander said.
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