One of the highest profile businessmen in Britain, 71-year-old Sorrell has run the company since 1986, taking it from a two-man operation in a London office to one that now dominates the industry with around 190,000 staff working in 112 countries.

The issue of who will replace Sorrell has long interested investors and on Friday the firm's chairman, Roberto Quarta, used his first annual report to update them after the report said last year that the process was becoming "steadily more rigorous and comprehensive".

"At some point we all leave our jobs," Quarta said. "The question is when.

"Whether, in Sir Martin's case, that happens tomorrow, in one, two, three, four or five years, or even over a longer period, we have already begun to identify internal and external candidates who should be considered."

As part of the succession process, Quarta said the chairman and independent board members had for some time met with senior staff to discuss who could succeed their chief executive.

When asked about his future, Sorrell tends to say that he will stay for as long as the firm wants him.

The owner of agencies including Ogilvy & Mather and JWT, WPP has outperformed its rivals in recent years due to its broad geographic spread and experience in digital marketing.

As a result, Sorrell was paid 70 million pounds in 2015 following the end of a now discontinued long-term incentive scheme.

With investors starting to rebel over the remuneration policies of many large firms, the company defended the package, saying the scheme had been approved by shareholders and reflected the long-term performance of the company.

"While the value of Sir Martin Sorrell's award is very large, it was the result of an outstanding set of returns to share owners," the report said.

The company said the incentive scheme had covered the period from 2011 to 2015 when WPP's share price rose by more than 95 percent, outperforming the wider FTSE by a large margin.

(Reporting by Kate Holton; Editing by Elaine Hardcastle)