Markit's and the Chartered Institute of Purchasing & Supply's services purchasing managers' index (PMI) for the U.K. dropped to 52.7 in February from 55.6 in January, missing expectations for a fall to 55.1. It was the lowest level since March 2013.
A reading above 50 indicates expansion in the sector.
The decrease was driven by a slower growth in new business and employment.
"The slowdown in February leaves the PMI surveys suggesting that economic growth could weaken to 0.3% in the first quarter, but there are downside risks to even this modest expansion," the Chief Economist at Markit Chris Williamson said.