WPP’s third quarter reported revenue is down 0.8% at £3.76bn, according to its trading update, while reported revenue for the first nine months of 2018 decreased by 1.6% to £11.25bn.

The company has also cut its full-year guidance, saying net sales are likely to be down 0.5% to 1% – the previous outlook was growth of 0.3%.

In a statement, chief executive Mark Read said there is a “significant opportunity” for Kantar to be developed into “the world’s leading data, insights and consulting company”, but said WPP needed to make “tough choices” over its future, with the best option being to sell a stake to a strategic or financial partner. 

He said: "The board has approved a formal process to review the strategic options that will maximise share owner value. It is envisaged that WPP will remain a share owner with strategic links to ensure that the benefits to clients are realised."

Read added that preparations, involving Kantar management, are underway, and that "unsolicited expressions of interest have been received".

Speculation over Kantar’s future has been mounting since WPP chief executive Martin Sorrell’s high-profile sudden departure earlier in the year as the company looks to improve its balance sheet. It has previously been estimated that Kantar’s sale could raise £3.5bn for WPP.

In April, it was reported that Kantar chief executive Eric Salama was seeking backers who might support a management buyout.

WPP’s data investment management business improved slightly in the third quarter, with like-for-like revenue less pass-through costs down 1.2% compared with -1.3% in the second quarter. While Asia-Pacific and Western Europe showed improvement, the US and UK were challenged as Kantar Insights and Kantar Media came under pressure.

WPP's share price dropped by over 20% following the third quarter update.