A Tory donor has warned he will stop funding the party if Theresa May's Brexit plans mean taking the UK out of the single market.

Engineering firm chairman, Sir Andrew Cook, who has given more than £1.2 million to the party, said the UK could "sleepwalk to disaster" if it left the single market in order to control immigration from the EU.

He told The Times (£) he would find it impossible to continue funding the party if the PM endorsed the move.

"It seems to me the political tail is wagging the economic dog," he said. "There appears to be a willingness to consider the sacrifice of withdrawal from the single market, which I believe will be a catastrophe."

The William Cook boss said that the economic arguments of staying in the single market were "overwhelming", adding that at least one of his factories was almost "entirely dependent" on access to the European market.

Sir Andrew was treasurer for the Conservative 'In' campaign and was handed a knighthood in David Cameron's resignation honours list. Credit: PA

Sir Andrew backed the Remain cause in the 2016 referendum and acted as treasurer for the Conservative In campaign.

He was awarded a knighthood in David Cameron's resignation honours list.

But he said he would "find it impossible" to keep funding the party if May backed leaving the single market.

"It is very difficult to make a political donation to a party when, although I support it ideologically, I do not believe that my interests and my ideology are ad idem with the principal Brexiteers," he added.

The PM has insisted she wants to have the "maximum freedom to trade with and operate in the single market" but her desire to secure curbs on immigration suggests that the UK will have to give up its full membership.

EU leaders have stressed that free movement of its citizens is a condition of single market membership that will not be negotiated away.