Markit's and the Chartered Institute of Purchasing & Supply's construction purchasing managers' index (PMI) for the U.K. dropped to 55.0 in January from 57.8 in December, missing expectations for a decrease to 57.5. It was the lowest level since April 2015.
A reading above 50 indicates expansion in the construction sector.
The index was driven by a softer growth in new business, while the employment rose was at the weakest pace since September 2013.
"UK construction firms struggled for momentum at the start of this year, with heightened economic uncertainty acting as a brake on new orders and contributing to one of the weakest rises in output levels since the summer of 2013. Softer growth of house building activity and a more subdued increase in commercial construction were the main factors behind the slowdown," Senior Economist at Markit, Tim Moore, said.