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Markit's and the Chartered Institute of Purchasing & Supply's services purchasing managers' index (PMI) for the U.K. rose to 55.6 in January from 55.5 in December, beating expectations for a fall to 55.3.
A reading above 50 indicates expansion in the sector.
The increase was driven by a faster growth in new business and job creation.
"The economy defied expectations and picked up speed in January, but cracks continue to appear in the country's resilience to the various headwinds," the Chief Economist at Markit Chris Williamson said.
He added that the U.K. economy is expected to expand 0.6% in the first quarter of 2016.