The British services purchasing managers' index (PMI) fell to 57.7 in June from 58.6 in May, marking a three-month low, Markit said Thursday.
The services sector contributes around three quarters of Britain's GDP, according to the Office of National Statistics (ONS).
Despite the disappointing headline number, June's survey signaled the sharpest increase in new business volumes for six months
The data marked the eighteenth consecutive rise in new jobs, with panelists reporting trading conditions remained positive and market confidence high in June, said the London-based survey compiler.
Another substantial increase in new jobs in June placed additional pressure on capacity levels.
Volumes of work outstanding held by British service providers increased at the strongest rate for four months, with growth also well above the survey's historical average, said Markit.
Markit's chief economist Chris Williamson said today's reports signaled that the momentum was holding after a strong performance by Britain's economy this year.
"Alongside an ongoing surge in construction and the largest quarterly rise in manufacturing output for 20 years, the services PMI confirms that the economy is firing on all cylinders," said Williamson.
"We expect the economy to grow by 0.8 percent again in the second quarter, taking GDP to a new all-time high," he added.
"The UK services sector, alongside strong performances from manufacturing and construction, has cemented expectations that the economic recovery can power ahead into the second half of the year," said David Noble, group CEO.
The Bank of England gave mixed signals on the time of interest-rate hike recently. The strong data has reinforced expectations that the central bank may act to change rates this year.