WPP’s Sir Martin Sorrell has said that British businesses have to show a “stiff upper lip and make the best of it” amid reports warning of a huge decline in advertising spend following the UK vote to leave the European Union.

The chief executive of the world’s largest marketing service group spoke as UK media stocks saw falls of up to 20% as jittery investors worried about the potential impact of Brexit.

Sorrell, a fervent remain supporter, said he was “very disappointed” by the outcome of the referendum said that there will now be “considerable uncertainty” for businesses.

“This is not good news, to say the least,” he said. “The PM’s resignation clearly adds to the uncertainty. However, we must deploy that upper lip and make the best of it. The resulting uncertainty, which will be considerable, will obviously slow decision-making and deter activity.”

WPP’s share price was down more than 5% as Sorrell pointed out that the company must look to build its presence in Europe even further following the Brexit decision.

“Four of WPP’s top 10 markets are in western continental Europe and we must build our presence there even further,” he said.

Earlier this week Group M, WPP’s global media arm, stripped £220m from its forecast for TV and newspaper ad spend in the UK over fears of a Brexit vote.

The biggest share price falls were felt by businesses that rely on the UK ad market, which has been thrown into a state of uncertainty following the news.

“Domestic ad-funded players will have a torrid time as fears on economic growth and business confidence will be seen to impact domestic ad spend,” said Malcolm Morgan and Ales DeGroote, analysts at Peel Hunt.

ITV, which remains dependent on the now jittery advertising market for the majority of its revenues, was one of the biggest fallers with its share price down about 20% in Friday morning trading.

The news prompted Adam Crozier, the broadcaster’s chief executive, to issue a statement to staff warning of “short term uncertainty” but said the business must stay the course.

“It has clearly been a huge night for the UK and events are still unfolding,” said Crozier. “But I want to reassure you that for us at ITV our strategy remains unchanged and we will continue to do what we do really well every day – which is to create and broadcast great programmes and sell them around the world.”

Rupert Murdoch-controlled Sky, which has operations in Germany, Italy and Austria, fell by 8% with analysts pointing out that it could be heavily affected as it makes almost 90% of its profits in the UK and Ireland.

Stock market-listed newspaper groups, which like ITV rely on advertisers staying the course following the vote, were particularly hard hit.

Daily Mail & General Trust, the owner of the Daily Mail, Mail Online and Mail on Sunday, fell 6%, Daily Mirror publisher Trinity Mirror is trading down 19% while Johnston Press, the owner of hundreds of regional and local titles and national newspaper i, dropped 17%.

“A Brexit vote is likely to lead to uncertainty and lower UK GDP growth impacting companies with material domestic UK exposure, such as ITV, Sky, DMGT, Trinity Mirror and Johnston Press which would all be heavily affected,” said Citi in a note to investors on Friday morning.

A “major blow” – or no immediate cause for concern?

There were warnings that the Brexit vote could have a “devastating” impact on the UK film and TV industry.

A big exporter of TV around the world, UK sales of shows and formats to Europe are worth about £380m a year.

International co-productions are now commonplace in the TV industry, with TV dramas made with a global audience in mind.

Michael Ryan, chairman of the Independent Film & Television Alliance, said the vote to leave the EU was a “major blow”.

“As of today, we no longer know how our relationships with co-producers, financiers and distributors will work,” he said. “The UK creative sector has been a strong and vibrant contributor to the economy. This is likely to be devastating for us.”

The UK sector has benefited to the tune of £78m between 2007 and 2013 in EU funding for shows such as BBC1’s Jamaica Inn, the BBC and S4C’s detective drama Hinterland and Sky Atlantic’s The Last Panthers, starring John Hurt.

The EU’s media programme, part of its Creative Europe scheme, has also part funded children’s shows such as Shaun the Sheep and Julia Donaldson’s Room on the Broom, as well as films Carol, Mr Turner, and Amy Winehouse biopic, Amy.

Pact, the independent producers’ trade body, said it was “disappointed” with the decision but said there were no “immediate causes for concern”.

Pact chair Laura Mansfield said it would be “working closely with both the UK government to make sure that necessary trade deals with Europe are secured as quickly as possible, and will continue to influence the [European] Commission’s proposals around the digital single market”.