Sorrell has denied that the allegations have merit.
The ad business has changed a lot since Sorrell launched WPP.
"As I look ahead, I see that the current disruption is simply putting too much unnecessary pressure on the business", Sorrell said in a statement to WPP staff.
"I will particularly miss the daily interactions with everyone across the world and want to thank them and their families for all they have done, and will do, for WPP", Sorrell added. "Good fortune and Godspeed to all of you ... now Back to the Future".
Mr Sorrell is among the most influential advertising professionals globally and also the longest-serving FTSE-100 chief executive, who has run WPP since setting up the business in 1985.
Details of the investigation into the allegation of misconduct were still not clear at the time of writing, with WPP having only offer the following update that it had concluded that that "the allegation did not involve amounts that are material".
"WPP is investigating an allegation of financial impropriety by me, specifically as to the use of company funds", Mr Sorrell said in a statement.
If Mr. Sorrell departs, the board is considering tapping WPP veterans Mark Read and Andrew Scott to be co-CEOs, the people said.
The resignation preempts what could have been an inglorious departure.
WPP stated that the amounts involved in Sorrell's alleged misuse of company funds were "not material" to the company, leading one United Kingdom agency chief executive to question the board's intentions. "However, there will be great interest in resolving the full-time CEO role as soon as possible". He worked closely with the brothers Saatchi (Charles and Maurice) and helped fashion its rapid growth, before going solo and acquiring a shopping basket manufacturer Wires & Plastic Products (WPP).
He owns shares in WPP worth more than £200m, while his annual remuneration packages - he earned £70m two years ago - have made him a target for critics of lavish boardroom pay.
But beyond the investigation that leaked this month, there were other signs Sorrell was losing his magic touch.
The probe served to intensify scrutiny on Sorrell and WPP's succession planning, long criticised by investors as being insufficient, prompting speculation that a CEO long seen as unstoppable might be forced to step aside. He will be treated as having retired on leaving WPP, as detailed in the Directors' Compensation Policy.
North Korea and China seem to be celebrating closer ties, with Kim Jong Un putting on a banquet today . Song and an art troupe to Pyongyang, the North Korean capital, to attend an worldwide art festival.
Red Bull's Daniel Ricciardo battles with Nico Hulkenberg . "I feel like every win I've had it's been a similar circumstance". Vettel and teammate Raikkonen grabbed a lockout for Ferrari on the front row to put the pressure on Mercedes .
What to make of Paul Pogba? It was a set piece and a lovely goal from Rodriguez so thank you. 20th, with 24 points from 34 games. Here's the best of the social media reaction to City winning the Premier League for the third time in the club's history.
Tristram Hunt, the former MP and current director of the V&A, described it as a 'remarkable extension of French cultural prowess'. Ali Mayhoub, who read the statement on Syrian TV, said "our air defenses effectively shot down most of them".
He also recalls Trump requesting that the FBI investigate salacious allegations about his actions with Russian prostitutes. Russian Federation has denied interfering in the election and Trump has denied any collusion or improper activity.
Four years ago, Beyoncé surprised her little sister Solange at Coachella for an adorable, impromptu dance . They were nominated for 14 Grammy Awards - winning twice for best R&B group performance and best R&B song.
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Barbara Bush in failing health: statement
Barbara Bush is in failing health and will not seek additional medical treatment, the Bush family spokesman announced Sunday. Granddaughter Jenna Bush Hager posted a photo to Instagram earlier this week showing the four generations of Bush women.
Anticipated $1.97 EPS for S&P Global Inc
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White Sox Game Postponed Due To Snow
Berrios allowed three hits and struck out 11 in seven innings Thursday as he continues to develop into the Twins ace. It's the first time in the history of Target Field that consecutive games have been postponed due to weather.