WPP plc (WPP) - Get Report shares plunged the most in more than 20 years Thursday after the world's biggest advertizing group posted its weakest full-year sales since the financial crisis and warned that "fundamental changes" in the industry would likely mean little growth across the whole of 2018.

WPP said it would focus on restructuring its sprawling global business of digital and traditional agencies into a more comprehensive unit as it rides out a year in which it sees only flat sales and modest improvements on margins. Net sales for 2017 fell 0.9%, the group said, while like-for-like revenues fell 0.3%. The group, led by Sir Martin Sorrell, also lowered its longer-term earnings per share growth forecast to between 5% and 10%, nearly have the pace it expected this time last year.

"The major factors influencing this performance were probably the long-term impact of technological disruption and more the short-term focus of zero-based budgeters, activist investors and private equity than, we believe, the suggested disintermediation of agencies by Google and Facebook or digital competition from consultants," Sorrell said.

"In this environment, the most successful agency groups will be those who offer simplicity and flexibility of structure to deliver efficient, effective solutions - and therefore growth - for their clients. With this in mind, we are now accelerating the implementation of our strategy for the group."

WPP shares fell more than 14.8% in the opening 90 minutes of in London, the biggest single-day decline since 1998, before being halted by regulators at a three-and-a-half-year low of 1,189.5 pence each.

Shares finished the session down 8.3%.

The gloomy assessment hit European media stocks across the board, dragging French rival Publicis SA  (PUBGY) falling 3.77% in Paris to change hands at €59.66 each in the opening hour of trading.

WPP's challenges reflect massive changes in the global advertizing market, where digital spending topped traditional TV ad buys -- by $209 billion to $178 billion -- for the first time last year, according to figures from IPG Mediabrand's Magna research division.

Magna said digital spends would grow 13% this year, more than five times the advance of television, and will likely comprise 50% of the total market by 2020.