Back in February 2010, a few months before he entered the Treasury, George Osborne declared: "Our first benchmark is to cut the deficit more quickly to safeguard Britain’s credit rating. I know that we are taking a political gamble to set this up as a measure of success." A gamble it was and how it has backfired on the Chancellor. Tonight, Moody's became the first rating agency to strip the UK of its AAA credit rating (downgrading it to AA1), citing the "continuing weakness" in the UK's growth outlook and its "high and rising debt burden".

For Osborne, who chose to make our credit rating the ultimate metric of economic stability, it is a humiliating moment. Not my words, but his. During one of his rhetorical assaults against Labour in August 2009, he warned: "Britain faces the humiliating possibility of losing its international credit rating". Rarely before or after becoming Chancellor, did Osborne miss an opportunity to remind us just how important he thought the retention of our AAA rating was. When the UK was first put on negative outlook by Standard & Poor's, he said:

It's now clear that Britain's economic reputation is on the line at the next general election, another reason for bringing the date forward and having that election now ... For the first time since these ratings began in 1978, the outlook for British debt has been downgraded from stable to negative.

After it was later moved off negative outlook, he declared:

Last April, the absence of a credible deficit plan meant our country's credit rating was on negative outlook and our market interest rates were higher than Italy's.

By Osborne's own logic, then, his deficit plan is no longer credible.

Tonight, the Chancellor has, unsurprisingly, described the decision as "a stark reminder of the debt problems facing our country – and the clearest possible warning to anyone who thinks we can run away from dealing with those problems". His cause is aided by the fact that the hawkish rating agencies want more austerity, not less. In its explanation of the decision, Moody's cited "reduced political commitment to fiscal consolidation". As he comes under attack from Labour, Osborne will retort, "but you want to borrow even more!" Ed Miliband and Ed Balls, who frequently shy away from making the explicit case for Keynesian stimulus, will need a clear and strong response.

The economic consequences of the downgrade are unlikely to be significant. France and the US, for instance, have seen no rise in their borrowing costs since losing their AAA ratings (in fact, yields on US and French bonds have fallen). All the evidence we have suggests that the market is prepared to lend to countries that can borrow in their own currencies (such as the UK) and that enjoy the benefits of an independent monetary policy, regardless of their credit ratings or their debt levels. But the politics of the downgrade are toxic for Osborne.

Still, you might ask, why should we listen to Moody's, the agency that gave AIG an AAA rating just a month before it collapsed? The answer is simple: we shouldn't. But this doesn't alter the fact that Osborne did. For political purposes, he used Britain's credit rating as a stick to beat Labour with. He can hardly complain if others now use this move against him. Tonight, the Chancellor has been hoist with his own petard.