UPDATE 3-WPP shares plunge as falls behind ad rivals in post-Sorrell era

* Cuts sales and profit full-year outlook

* Quarterly sales dropped sharply

* Shares down 20 percent

* New boss will sell stake in Kantar (Adds shares, background)

LONDON, Oct 25 (Reuters) - WPP lost a fifth of its market value on Thursday after a major downturn at its creative agencies in New York and London forced it to cut its sales and profit forecasts, ramping up the pressure on new boss Mark Read.

The share fall wiped 2.8 billion pounds ($3.6 billion) off WPP's market capitalisation, taking the stock to a six-year low.

Read, who inherited the world's biggest advertising group when its founder Martin Sorrell left abruptly in April, said WPP needed to sell assets and hold off acquisitions to fund an investment drive for more talent at its agencies such as JWT, Ogilvy and Y&R.

The British group, which will start by selling a stake in data analytics group Kantar, stood in contrast to its peers Omnicom, IPG and Publicis which have all reported relatively stable trading of late.

WPP shares were down 20 percent at 0715 GMT after it said third quarter net sales fell 1.5 percent, compared with a rise of 0.7 percent in the previous three months.

As a result the group lowered its net sales full-year guidance, saying it could fall as much as 1 percent compared with a target of 0.3 percent growth just three months ago.

The operating margin is likely to be down between 1 and 1.5 margin points, compared with a previous prediction of down 0.4 percent.

WPP, a holding company built by Sorrell to provide creative agencies, media planning, public relations, consultancy and data analytics, has been struggling since the start of 2017 when it suddenly started to lose accounts.

Sorrell left following a complaint about his personal conduct, which he denied.

The group faces pressures from new competitors such as consultants Deloitte and Accenture and the tech giants Facebook and Google, while major clients such as Unilever are cutting spending and creating some work in house themselves.

In recent months it has also lost some major pitches, most notably the creative account for Ford to Omnicom's BBDO, which it had previously held for 75 years.

"Turning around WPP requires decisive action and radical thinking, and our performance in the third quarter of 2018 reinforces our belief in that approach," Read said.

WPP also said that Finance Director Paul Richardson would step down after 22 years in the role. ($1 = 0.7748 pounds) (Reporting by Kate Holton; editing by Guy Faulconbridge and Alexdander Smith)