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Shares in WPP have closed down nearly 14% after the advertising giant warned that growth in the business is slowing.

The firm, whose former boss Sir Martin Sorrell stepped down in April after a scandal, reported lower-than-expected third quarter sales and slashed its full-year outlook.

The world's biggest adverting firm is overhauling its business amid mounting competition from Google and Facebook.

New boss Mark Read called for "radical thinking" to turn the company around.

"Our industry is facing structural change, not structural decline, but in the past we have been too slow to adapt, become too complicated and have under-invested in core parts of our business," he said.

"There is much to do and we have taken a number of critical actions to address these legacy issues and improve our performance."

WPP said total group revenue fell by 0.8% in the three months to 30 September, as demand at its US and UK creative agencies dived.

The British firm - which owns big-named agencies such as Ogilvy and JWT - said it would sell off more assets and hold back from making new acquisitions.

Mr Read said WPP would offload its stake in data analytics group Kantar, which had revenues of £2.7bn last year. "Preparations are underway, involving Kantar management, and unsolicited expressions of interest have been received," Mr Read said.

This would allow it to pump cash into its businesses, particularly in its key North American business where sales are down 5.8% this year.

Founded as a holding company in 1986, WPP's operations today span creative agencies, public relations, consultancy and data analytics.

However, it has been struggling over the last few years amid mounting competition from tech platforms Facebook and Google as more advertising moves online.

Traditional consultancies such as Deloitte are also entering the market while major clients such as Unilever are spending less on advertising.

That said, WPP's results come after its peers - Omnicom, Interpublic and Publicis - all published third quarter updates that were well received.

WPP said its sales could fall as much as 1% in 2018 compared with a target of 0.3% growth just three months ago.

George Salmon, equity analyst at Hargreaves Lansdown, said: "We're yet to get the full details, but it looks like the over-riding theme of [Mark Read's] restructure will be a simplification of the business. It's easy to see why.

"Taking over at a group where success depends so much on having an in-depth knowledge of all the various agencies and divisions was always going to be a serious challenge.

"This journey has already started, and the decision to sell a stake in Kantar is the next step."

Sir Martin, who took WPP from a small engineering company and created the world's biggest ad agency, quit this year after facing a complaint of personal misconduct - which he vigorously denied.

He has since formed rival venture S4 Capital and in July outbid his former company to buy the Dutch digital production company MediaMonks.