1978, and from Fitch Ratings since 1994.
Moody's said that despite considerable economic strengths, Britain's growth was likely to be sluggish due to a mix of weaker global economic activity - especially in the euro zone - and a drag "from the ongoing domestic public and private-sector de-leveraging process."
"This period of sluggish growth poses challenges to the government's fiscal consolidation program, which we now assume will extend well into the next parliament," Moody's analyst Sarah Carlson said in a telephone interview with Reuters.
But Ed Balls, the Labour Party's main spokesman on finance issues, said the Moody's decision should be a wake-up call for Osborne ahead of his annual budget statement as Chancellor of the Exchequer.
"This credit rating downgrade is a humiliating blow to a Prime Minister and Chancellor who said keeping our AAA rating was the test of their economic and political credibility."
"The issue is no longer whether this Chancellor can admit his mistakes but whether the Prime Minister can now see that, with UK economic policy so badly downgraded in every sense, things have got to change."
Howard Archer, chief UK economist at IHS Global Insight, said a new approach from Osborne was improbable.
"The strong likelihood is though that it will not materially lead to a change in his plans."
Changes are more likely from the Bank of England, which surprised markets earlier this week after it revealed that Governor Mervyn King and two other policymakers favoured restarting bond purchases to boost the economy.
They remained in the minority among their fellow policymakers but economists increasingly expect more stimulus eventually by the central bank.
This - and the central bank's tolerance of above-target inflation - have combined to put pressure on sterling while leaving British government debt relatively shielded.
Charles Diebel, a fixed income strategist at British bank Lloyds, was sanguine about the impact of the downgrade on gilts, as U.S. and French debt was not badly affected when these countries lost their triple-A ratings.
"This has been speculated as inevitable and is most likely largely in the market. I would expect only very limited damage to the gilt curve and to sterling. Historically, losing your AAA is actually a bond bullish event," he said.