The PMI picked up to 53.6 in September, slightly better than expectations in a Reuters poll of economists for it to hold at August's level of 53.2.
Any score of above 50 indicates growth, so this figure shows a steady expansion.
Sterling rose after the services PMI report, gaining about 0.4 percent.
Markit's final composite PMI, which tracks the manufacturing and services sectors that account for more than two-thirds of the economy, rose to 57.7-its highest reading in nearly 6-1/2 years-from 55.8 in August.
At 3.45 am ET, IHS Markit is slated to release Italy's services PMI data. While the positive figure provided an immediate boost to the pound, which jumped to $1.3285 in forex markets after the data, from a session low of $1.3242, it does little to change the near-term outlook for the United Kingdom economy.
Ireland was in second position-despite seeing growth slip to a two-month low-while the revival in France gathered pace.
The pound rose this morning after new data showed the services sector beat expectations in September, becoming the only one of the UK's three biggest sectors not to disappoint analysts.
Services firms also reported a decline in optimism over growth for the year ahead.
Continuing uncertainty over the economy in the wake of the Brexit vote has been blamed for a shock fall in construction activity. New business orders were also down, with house building being the only bright spot.
The report paints a mixed picture for the Bank of England, where policy makers have turned hawkish and indicated they may be close to raising interest rates.
In contrast to the UK's flagging performance, the 19 countries in the eurozone continued to expand at a strong pace.
USA services sector activity strengthened further in September, a closely followed survey indicated on Wednesday, with a slight easing in the rate of growth that was expected due to disruption caused by hurricanes Harvey and Irma. The euro fell to 0.8850 pounds after today's strong United Kingdom services PMI, reversing the small gains made a short while earlier from the release of the Eurozone PMIs. Sales are forecast to grow 4.4 percent on year in August, following a 4.2 percent rise in July.