LONDON, July 19 (Reuters) - Martin Sorrell's S4 Capital has increased its firepower for acquisitions after the advertising veteran saw the biggest jump in activity in his 45-year career, driven in part by companies converting their operations to digital.

The British company, which has a market valuation of 3.6 billion pounds ($5 billion) less than three years after it launched, said on Monday it had refinanced its debt to provide around 200 million pounds for purposes including deals.

Sorrell said he was planning to expand into technology services to better compete with the likes of Accenture and Globant which can help clients develop IT and internal operations and market them online for digital sales.

The expansion would enable S4 to win work from the chief technology and information officers of clients, as well as the chief marketing officer.

The willingness to expand comes as advertising groups benefit from a rebound in global economic growth as pandemic-linked restrictions ease, and as companies continue a shift towards digitizing their operations. As a digital-only marketing group, S4 was in prime position, Sorrell said.

"Organically we're doing very well, it's beyond expectations certainly through June, it's unprecedented from what I can see in the 45 years that I've been at it," he told Reuters.

The world's most famous advertising executive spent most of his career making acquisitions, first at Saatchi & Saatchi and then as he built WPP into the world's biggest advertising group.

His new firm already counts Google and Facebook as clients, and S4 said on Monday its organic revenue and gross profit had continued to grow at levels beyond expectations.

Shares in the company were down 2%, broadly in line with a weaker FTSE.

($1 = 0.7261 pounds) (Reporting by Kate Holton; editing by Sarah Young and Bernadette Baum)