Markit's and the Chartered Institute of Purchasing & Supply's services purchasing managers' index (PMI) for the U.K. declined to 55.5 in December from 55.9 in November, missing expectations for a fall to 55.6.
A reading above 50 indicates expansion in the sector.
The decline was driven by a slower growth in new business and job creation.
"The services sector remained the key driver of the UK's economic upturn in December, helping to offset the recent weakness seen in manufacturing and putting the economy on the starting block for another year of 2-2.5% growth in 2016," the Chief Economist at Markit Chris Williamson said.
He added that the U.K. economy expanded 0.5% in the fourth quarter of 2015.