Institutional Shareholder Services (ISS) has advised WPP's shareholders to approve Sorrell's pay at the company's annual meeting on June 8, saying a large chunk of the pay is related to WPP's performance during the year.

ISS said: "We highlight that the bulk of the CEO's 2015 headline realised pay number is largely driven by his participation in ... a plan which shareholders approved by a comfortable margin ... in 2009 and in part reflects the company's run of extended strong performance."

"The company has also outperformed its global peers on a relative basis, as required by the performance conditions of the scheme," ISS said in a statement.

WPP, the world's biggest advertising agency, declined to comment.

WPP, which owns advertising companies like JWT and Ogilvy & Mather, has clashed with investors before over the size of Sorrell's pay deal.

In 2012, a majority of WPP shareholders voted against Sorrell's pay, but last year Sorrell saw off a potential shareholder revolt over his 43 million pound pay for 2014.

He has built WPP from a two-man operation in a London office to one that now dominates the industry with around 190,000 staff in 112 countries.

The company has outperformed rivals in recent years due to its geographical spread and digital marketing experience.

Other big British companies have faced shareholder opposition to executive pay packages this year in a resurgence of investor activism against excessive boardroom salaries.

Last month, shareholders in BP voted against Chief Executive Bob Dudley's $20 million pay deal for 2015 after it made a record annual loss.

Shareholders in Scottish engineering Weir Group and medical equipment firm Smith & Nephew also rejected their 2015 payouts in non-binding votes.

A recent Reuters analysis of executive pay at Britain's top companies showed that the average pay of CEOs fell in 2015, but a deeper slide in corporate profits meant their cut of the spoils continued a decade-long rising trend.

(Reporting by Rahul B in Bengaluru; Editing by Jane Merriman)