The world's biggest advertising firm is reporting signs of "Brexit anxiety."

WPP said almost all its British businesses reported slower growth in the third quarter, with sales up 2.7%, compared with 3.4% in the previous quarter.

"Perhaps, the first signs of Brexit anxiety," the company said in a statement.

WPP's CEO, Martin Sorrell, had campaigned against Britain's decision to leave the European Union. He said the uncertainty could be a "growth killer."

WPP said global revenue was up 23.4% at £3.6 billion ($4.4 billion) in the third quarter compared with the same period last year. WPP (WPPGF) stock jumped 3.5% in early trading in London.

The company said sales growth is down partly because of the collapsing value of the pound since the Brexit referendum in June.

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The currency has lost nearly 19% since Britain voted to leave the EU. Most of WPP's sales are outside the U.K., and the company benefits from the cheaper pound because its services are cheaper for customers abroad.

But in the long term, "it's going to make Britain a tougher place to trade," Sorrell told CNNMoney.

"I tend to look at the currency as being the country's stock price," he said. "And effectively, the U.K.'s stock price is down by 15% to 20%, and we are going to have to pay for that in terms of increased cost of imports."

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The government has started to unveil its Brexit plans in the last few weeks. Talks with the EU will begin before March, and the exit itself will happen two years later.

There are still no details about how the government plans to approach the negotiation and what kind of trade deals will it seek.