Sorrell was speaking at the Confederation of British Industry's annual conference in London yesterday (21 November), and told the 1,000-strong audience: "I don't care what people say about Brexit, the vote to leave the European Union has increased levels of uncertainty among British businesses."
But Sorrell said that despite the Government's need "to be flexible" in its negotiations, businesses needed to have more certainty about what Brexit would actually mean in pracitce.
He also said Brexit was creating anxiety among employees from EU member countries.
WPP employs roughly 200,000 people globally, and about 17,000 in the UK - with around 15 per cent of the latter figure being EU nationals.
These workers are "extremely worried", Sorrell told delegates, particularly in the aftermath of home secretary Amber Rudd's suggestion that the Government might force British businesses to reveal the number of foreign people they employ - even if this was abandoned.
He also said the election of Donald Trump as the next US President had caused uncertainty and concern on a global scale.
Sorrell suggested it was hard to know "what sort of Donald Trump we will get" - but added that it was bound to affect the UK.
"We have no basis of understanding what Trump will actually do. He likes to confound the cynics and he may turn out to be the best thing since sliced bread... but we really don't know."
Sorrell did, however, suggest that Trump could be good for business in the short to medium term.