A major 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.
Sir Andrew Cook, who has given more than £1.2million to the party, warned the country could "sleepwalk to disaster" if it sacrificed single market membership in order to control immigration from the European Union.
The engineering firm chairman told The Times he would find it impossible to continue donating to the Conservatives if the Prime Minister endorsed the move.
He said the "economic arguments of staying in the single market are overwhelming" but political pressure was leading to the potential "catastrophe" of leaving it.
"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."
Sir Andrew, who is chairman of William Cook, an engineering firm producing components for applications including rail, energy and defence, told the newspaper that at least one of his factories was almost "entirely dependent" on access to the European market.
If the Tory leadership backed leaving the single market, he said he would "find it impossible under those circumstances" to keep funding the party.
"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."
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.
The Prime Minister has insisted she wants firms to have the "maximum freedom to trade with and operate in the single market", her desire to secure curbs on immigration suggests that the UK will have to give up its full membership.
EU leaders have repeatedly stressed that free movement of its citizens is a condition of single market membership that will not be negotiated away.