Markit's and the Chartered Institute of Purchasing & Supply's services purchasing managers' index (PMI) for the U.K. fell to 53.3 in September from 55.6 in August, missing expectations for a rise to 56.0. It was the lowest level since April 2013.
A reading above 50 indicates expansion in the sector.
The decline was partly driven by a slower growth in new business.
"Weakness is spreading from the struggling manufacturing sector, hitting transport and other industrial-related services in particular. There are also signs that consumers have become more cautious and are pulling back on their leisure spending, such as on restaurants and hotels," the Chief Economist at Markit Chris Williamson said.