The UK's service sector stagnated last month, with new orders falling for the first time in two-and-a-half years, according to the IHS Markit/CIPS purchasing managers' index (PMI).
The figures showed a reading of 50.1 in January, lower than December's 51.2.
IHS Markit's Chris Williamson said the results indicated that the UK economy "is at risk of stalling or worse".
He said this was because of growing Brexit uncertainty coinciding with a wider slowdown in the global economy.
Economists had expected a reading of 51 - a figure above 50 indicates growth.
It follows last Friday's news that the PMI figures for manufacturing fell to 52.8 last month - the second weakest reading since July 2016.
The figures revealed that UK manufacturers were preparing for Brexit by stockpiling raw materials at a record pace and that there was a risk of the sector slipping into recession.