The headline index, marking growth in the sector with figures above 50, rose from 56.9 in June to 60.2, the highest level since December 2006.
The index rounds out complimentary indexes for manufacturing and construction, Markit senior economist Paul Smith said.
"Strong Services PMI data for July follows on from equally positive numbers from sister construction and manufacturing surveys," Smith said.
Smith said the recovery was becoming "increasingly broad-based."
"This is the clearest sign yet that the U.K. economy ... has the momentum to deliver continued growth," Markit Chief Economist David Noble said.