Global advertising group WPP may face opposition from shareholders at its annual meeting on 9 June over chief executive Martin Sorrell's £43m pay award for 2014, according to press reports.Glass Lewis and Institutional Shareholder Services (ISS), which advise investors that own about a quarter of the equities on the UK stock market, have both raised concerns about the 40% increase that Sir Martin received for 2014, the Financial Times reported.Shareholders have regularly protested against Sorrell's pay in the past few years. In 2012, 60% of shares were voted against his award and the company replaced his long-term incentive scheme with a less generous package in 2013.This year, Glass Lewis recommended that shareholders vote against Sorrell's remuneration - which would make him the best-paid boss on the FTSE 100 - saying it considers the award "wholly excessive" as it "far exceeds the compensation given to chief executives at similar firms".While it did not recommend a 'no' vote, ISS said the "overall quantum of Sir Martin Sorrell's pay remains exceptionally high". It said it supported the pay award because the remuneration scheme was performance-related and much of the increase was attributable to share price appreciation, the FT reported."The company's performance record is strong, with 2014 another record year by virtually any metric, including revenue, profitability, net sales margins, earnings per share and share price," ISS said.