S&P said it could lower the UK's rating "if fiscal performance weakens beyond our current expectations".
Fellow agencies Moody's and Fitch both revised the UK to negative outlook in the first half of this year, the BBC reports.
The Treasury pointed out that S&P endorsed the government's "strong commitment" to reducing the deficit.
"It is a hard road, but the economy is on the right track and just this week it was again confirmed that jobs are being created, with over one million private sector new jobs in the last two years," a Treasury spokesperson said.
Has been cold-calling consumers
New shares admitted to London Stock Exchange
Slow and steady growth
Missed funding target by £240,000
Denies any wrongdoing