Almost two-thirds of European Union businesses that work with U.K. suppliers plan to move some of their supply chain out of Britain because of its withdrawal from the bloc, according to an industry group.

That's an increase from May, when 44% of EU businesses expected to pull back from the U.K., according to a survey the Chartered Institute of Procurement & Supply released Monday. The group also found that 40% of U.K. businesses with EU suppliers have begun looking for domestic companies to replace them, up from 31% in May.

The findings underscore the concern among executives that the lack of an agreement on the terms of a British exit is hurting businesses. Prime Minister Theresa May will tell one of the nation's biggest business groups that she backs a transition period to give companies more certainty, while urging executives to be more optimistic about the process.