The Association of British Insurers has given its members a ‘warning' to encourage them to look closely at the scheme - known as Leadership Equity Acquisition Plan III (Leap) - before approving it.
The scheme will be put to a shareholder vote at WPP's annual general meeting in Ireland on June 2.
The ABI insisted it had not recommended that shareholders vote down the scheme, just that its ‘unique' structure should be carefully considered. But the Pension Investment and Research Consultancy has pledged to oppose the plan.
The scheme would allow Sorrell, who earned £3.1m last year, to invest up to £12.5m over the next five years and recoup five times that if all targets are hit.
The Times quoted a WPP spokesperson who said that the five-year performance term ‘was longer than most companies' and the company would need to perform in the top 10 per cent of its peer group for Sir Martin to get the maximum payout.