Rival ratings agencies Moodys and Fitch both downgraded Britain from AAA earlier this year.
S&P affirmed the UKs long-term sovereign rating at AAA/A-1+ and said the government continued to benefit from its exceptional monetary flexibility.
The agency said that while Britains economy was recovering on the back of private consumption and residential investment, the outlook remained negative, reflecting risks to the sustainability of economic growth.
The Moodys and Fitch downgrades were a blow to
finance minister George Osborne, whose refusal to ease back on his austerity drive to help cut the fiscal deficit has been somewhat vindicated by a turnaround in the economy.