Sir Martin Sorrell is in line for almost £20m in payouts from WPP over the next five years, as part of the deal struck to leave the advertising group he founded more than three decades ago.

The departure of Sorrell, who resigned on Saturday before learning the findings of an internal investigation into alleged personal misconduct, is also being viewed as a potential catalyst for a breakup of WPP.

The world’s largest advertising group said its founder and chief executive would be treated as having retired from the company. This means Sorrell will be eligible to receive payouts related to 1.6m shares in a number of award plans that will vest over the next five years. The exact value depends on the company’s performance. At WPP’s current share price of £12 the awards are worth about £19m.

WPP has said the investigation into Sorrell, 73, who rejected the allegation of misconduct “unreservedly”, has concluded. The law firm in charge of the investigation, WilmerHale, was due to deliver its findings to the board by next Friday but details of the misconduct allegation and the outcome of the probe will not be made public. The allegations are the improper use of company funds, and improper personal behaviour.

Sorrell’s final full-year pay as chief executive for 2017 will be made public when WPP publishes its annual report in about a month. He is thought to be receiving between £13m and £15m, the lower end of a maximum of £19m he could have earned due to the company reporting its worst performance since 2009.

Sorrell is one of the UK’s best-paid business leaders, earning more than £200m from pay and lucrative – and highly controversial – reward schemes in the past five years alone. His £70m payout in 2015, was one of the biggest in UK corporate history. Sorrell and his family trust also own a stake of about 1.8% in WPP, worth about £250m.

In an email to staff, Sorrell wrote: “I shall miss all of you greatly. As a founder, I can say that WPP is not just a matter of life or death, it was, is and will be more important than that.”

To fill the leadership vacuum while a successor is found, Roberto Quarta, the WPP chairman, will take on the role of executive chairman. In addition, Mark Read, the chief executive of Wunderman and WPP Digital, and Andrew Scott, a top executive at WPP’s European operation, have been appointed chief operating officers.

This would make them the frontrunning internal candidates to take over from Sorrell, as the board also readies to reach out to its “constantly refined list of external candidates”.

“The succession arrangements look like a sticking plaster and, until strong and credible long-term leadership arrangements emerge, there will be concerns about the sustainability of the WPP Group, as we know it today, and its strategic direction,” said Guy Jubb, former global head of governance and stewardship at Standard Life Investments, an investor in WPP.

Analysts believe Sorrell’s departure combined with a share price slump of almost a third over the past year means investors and the WPP board will have to consider that there is potentially more value in breaking up WPP.

“The question to Quarta is how do we create shareholder value from this position. I think most analysts and investors when adding up the sum of the parts of WPP would answer that it is to break it up,” said Alex DeGroote, an analyst with Cenkos Securities.

The company’s board is already running the rule over whether it should explore a sale of Kantar, its market research arm, which could be worth £3bn.

Beyond its marketing services and PR operations – of which WPP’s global media agency powerhouses such as MediaCom have been the engine of its profits and revenue growth – DeGroote says there is a “treasure trove of hidden value” on WPP’s balance sheet worth billions of pounds that is not reflected in its core profits.

He estimates there is £1bn of property and £1.1bn of equity investments in companies including Vice, digital firm AppNexus, measurement company comScore and advertising group Chime.