The reverberations from David Cameron’s speech are being felt here at the World Economic Forum in Davos.

The offer of a full in-out referendum was more than many businessman I’ve spoken to were expecting. The prime minister is in for a sharp reception from business leaders he is meeting here tomorrow.

Here’s why. WPP’s boss, Martin Sorrell, a fan of the PM, explained to me this morning how there had been a sense of optimism engendered by the recent raise in global stock markets. Finally, the $2tr sitting on global corporate balance sheets were being primed for deployment and investment, fostering growth and creating jobs.

Sorrell told me that he thought the PM’s speech this morning was like a “Grey Swan” (as in Nassim Taleb’s Black Swan – a bad shock to the system) alongside a Middle East crisis and the eurozone.

Sir Martin told me he “totally understands the PM’s predicament from a political point of view” and that he’s in a very difficult position with Ukip and the right wing of his party, and even that “you can understand that the PM wants a good open debate… if he can negotiate better terms all credit to him”.

But those were the caveats: “The problem is it does creates uncertainty”.

“At the very best you have to say its neutral at worst it’s negative… the outcome from the referendum decision and the speech this morning cannot in my view be positive for investment,” he said.

The speech had “injected uncertainty, and uncertainty is the last thing we need”. Mr Sorrell told me he had been speaking to global auto manufacturers at the Detroit Motor Show from South Korea, India, China, Japan and America who were anxious and “worried” about Britain’s EU membership question marks, and need certainty about long term investment decisions.

“They have raised concerns about investment prospects in Britain with this uncertainty”, Mr Sorrell told me.

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