A failure of the govenment to deal with the debt burden.
Moody's announced on Friday night that it had cut the Government's bond rating one notch from 'Aaa' - the highest possible level - to 'Aa1'.
He also took comfort that Moody's noted that "the UK's creditworthiness remains extremely high" thanks in part to a "strong track record of fiscal consolidation".
The move is a significant setback for Chancellor George Osborne, who has faced criticism that his strategy for dealing with UK's huge debt burden is failing to deliver.
Moody's pointed to "continuing weakness in the UK's medium-term growth outlook, with a period of sluggish growth which [it] now expects will extend into the second half of the decade".
The credit ratings agency also noted that the Government's debt reduction programme faced significant "challenges" and that the UK's huge debts are unlikely to "reverse before 2016".
Moody's said that despite considerable structural economic strengths, growth is expected to be sluggish due to a combination of weaker global economic activity and the drag on the UK economy "from the ongoing domestic public- and private-sector deleveraging process."
However, Moody's, which is the first ratings agency to lower the UK from the highest rating, said the outlook on UK debt is stable.
Mr Osborne responded to the downgrade by insisting he would not change course on the Government's austerity programme.
He called Moody's decision 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".
"Far from weakening our resolve to deliver our economic recovery plan, this decision redoubles it."
"We will go on delivering the plan that has cut the deficit by a quarter, and given us record low interest rates and record 'the UK's creditworthiness remains extremely high' thanks in part to a 'strong track record of fiscal consolidation' numbers of jobs."
Yes, it could happen here - again. The conditions that led to our first downgrade are present again, as the president refuses to deal with either our short term fiscal problems, or reform entitlements which is an essential step to fixing our long term debt problem.
The last time it happened, the president's approval rating tanked. One wonders if he will take heed of what has happened in Great Britain and act to avoid a similar downgrade in the US.