FILE PHOTO: Carolyn Fairbairn, Director General of the Confederation of British Industry (CBI), speaks at their annual conference in London, Britain, November 19, 2018. REUTERS/Toby Melville

LONDON (Reuters) - Leaving the European Union without a deal will cause severe long-term damage to Britain’s competitiveness, one of the country’s main business groups warned the 12 candidates hoping to succeed Theresa May as prime minister.

“Firms large and small are clear that leaving the EU with a deal is the best way forward,” the Director-General of the Confederation of British Industry (CBI) Carolyn Fairbairn wrote in a letter.

“Short-term disruption and long-term damage to British competitiveness will be severe if we leave without one.

“The vast majority of firms can never be prepared for no-deal, particularly our SME (small and medium-sized enterprises) members who cannot afford complex and costly contingency plans.”

Britain was due to leave the EU in March but that has been pushed back until Oct. 31 after lawmakers rejected May’s negotiated settlement.

A total of 12 Conservative lawmakers are seeking to take over from May with several saying they are prepared to take Britain out of the European Union without an agreement in place, which has further strained relations with firms.

“We have a clear message for the next Conservative leader and future Prime Minister. Champion business,” wrote Fairbairn.