Tesco's chairman John Allan has warned David Cameron that that major businesses could move their headquarters out of London if Britain leaves the EU.

John Allan, 66, said that the Tory leader's decision to promise an in-out referendum before knowing what powers can be clawed back from Brussels means "the cart is very firmly before the horse".

Allan – who has only recently been appointed the chairman of the supermarket giant after it revealed £3bn worth of losses – urged the prime minister that a better approach would be to seek reforms first before promising an in-out referendum.

Leaving UK is painless

Speaking to the Independent on Sunday, he claimed big companies like Tesco could move their head offices from London to other sites in Europe "relatively painlessly", should Britain exit the EU.

He said: "It's a question of weighing up the pluses and minuses. This issue of the EU referendum and the uncertainty that it creates and the possibility that it could lead to Britain leaving the EU is a particularly heavy kind of pebble to put in the scales."

The Conservative manifesto promises an in-out referendum by 2017 after they have negotiated reforms on Britain's membership.

The chairman, who described his political leaning as being "pretty close to the centre", was not among the more than 100 senior business leaders who signed an open letter last month praising the Tory government and warning that Labour could "put the recovery at risk".

Ukip 'spooking' parties

He said that he did not think it was sensible to "choose sides" as companies will have to work with whichever party won the elections, the paper reported.

Allan also warned the political parties to approach the issue of immigration carefully, saying they had been "spooked" by Ukip into making declarations on the issue.

"There are problems… that need to be sorted out, but I think the answer is not to lock the door and throw the key away," he said.

He also criticised Labour's key manifesto pledge of a mansion tax on homes worth over £2m, saying it was "pretty difficult to justify", the Independent on Sunday said.

He accused Ed Miliband's party of a lack of clarity on how the levy would work.