More British finance firms will move business to EU countries if Britain fails to hammer out a post-Brexit transition deal by the end of the year, London Stock Exchange chief Xavier Rolet warned on Wednesday.

“Unless on this side of Christmas a credible announcement is made, then my understanding is that in the first quarter of next year, the implementation stage will start,” Rolet said, referring to contingency arrangements being prepared by businesses.

“In the absence of certainty in the next few months, the businesses, the CEOs, the boards the executive committees of many companies that are based here will have to start acting on worst case scenarios,” he said at a meeting with MPs in parliament.

Prime Minister Theresa May has backed a two-year transition period to ease the impact on the economy but she is beset by divisions within her own Conservative party and talks with Brussels are stalled over the size of the UK’s exit bill.