Shareholders have once again placed pressure on the board of advertising giant WPP to resolve issues around what happens when current chief executive, Sir Martin Sorrell, leaves the company.

A letter from Standard Life Investment, which manages 19 million shares in WPP, to company chairman Roberto Quarta challenged the board to put in place measures to ensure an “orderly succession”, describing it as a “key risk”. The letter, from governance and stewardship director Deborah Gilshan, protests that Sir Martin’s contract has no notice period for departure and calls on the chairman to put one in place.

“Unusually, the CEO’s service contract may be terminated by either the company or Sir Martin without any notice,” the letter said. “Given this, we suggest that the board consider what lead time would be required to ensure an orderly succession and discuss this with Sir Martin.

“We would like the board to come to an agreement with him that, other things being equal, he will provide sufficient warning to meet this timeframe. In our opinion, this increases the likelihood of a seamless succession as it would allow the board to execute their succession plans effectively and would reduce further the risk profile of the company.”

Gilshan adds: “As another annual meeting passes, the time to address succession for the CEO shortens and the necessity to do so becomes more pressing.”