The UK's failure to meet a key public debt target "weakens the credibility" of its top AAA credit rating, the Fitch ratings agency has said.
Debt will now not fall as a proportion of the country's output until 2016-17, a year later than Chancellor George Osborne had targeted.
Fitch said that the Autumn Statement confirmed the scale of the challenge facing the UK.
In March, it said the UK's AAA rating was under threat.
A cut to the credit rating would mean that the country is perceived as more risky to lend to, thereby raising the cost of borrowing from international investors.
The Office for Budget Responsibility, the independent body that makes economic forecasts for the government, announced that the UK will miss its debt target and the economy will contract by 0.1% this year - a big revision from the time of the Budget in March, when it said that the economy would grow 0.8% this year.
Growth forecasts for the next five years were also cut.
"We forecast gross general government debt to peak at 97% in 2015-16, approaching the upper limit of the level consistent with the UK retaining its AAA status," Fitch said.
"The government has chosen not to chase the supplementary target by deploying additional consolidation measures over the next two years. In our view, missing the target weakens the credibility of the UK's fiscal framework, which is one of the factors supporting the [AAA] rating."
It warned in March that it could downgrade the UK in the next few years if the government does not contain the level of public debt.
Fitch said it would formally review the UK's rating after the next Budget in March 2013.
In February, rival agency Moody's also warned that the UK's credit rating may be cut in future, potentially increasing borrowing costs.
Confusion on borrowing
On borrowing figures, the chancellor said that debt would not begin to fall as a proportion of the country's output until 2016-17, which is a year later than the government's target.
Before the statement, many analysts had predicted that the budget deficit, which is the amount the government is having to borrow in the current year, would be higher than it was last year.
However, it is now forecast to fall from £121bn in 2011-12 to £108bn in 2012-13.
But there was some confusion about how that had been achieved, with shadow chancellor Ed Balls complaining about the full figures not being in the Mr Osborne's statement.
BBC economics editor Stephanie Flanders said that the deficit figure had fallen because the government had decided to use the proceeds from the sale of licences to run 4G mobile phone services to reduce this year's borrowing.
Without that, she said, the deficit would have risen "maybe by a couple of billion pounds".
There was also a reduction in the deficit of £11.5bn in the current year as a result of the Asset Purchase Facility.
As a result of the Bank of England's quantitative easing programme, the central bank currently owns a lot of the government's debt.
If anybody else had lent money to the government it would have had to pay interest on those loans.
The government has now decided it should not be paying interest to the Bank of England, and the benefit of that has reduced the deficit and will continue to do so for the next four years.