CBI boss Carolyn Fairbairn has issued a warning against "crash-landing" into Brexit without planning or sufficient preparation.
Ahead of Theresa May's landmark address on tomorrow, Fairbairn has issued a last-ditch plea for a negotiated settlement with Europe, including provisions for transition.
"The practicalities of a disorderly crash-landing need to be understood," Fairbairn told the Guardian.
"An exit into [World Trade Organisation rules] at the stroke of midnight without the proper planning and preparation in place would be very serious for the UK economy," she said.
"There are some signs that there is more conversation around that being an outcome. Our job is to demonstrate how difficult that would be because of all these unanswered questions," she added.
In making her speech tomorrow, May is expected to warn that the UK is prepared to quit both the Single Market and the European customs union.
Leading voices in the Square Mile have told City A.M. that the UK's financial services sector has been placated by recent comments from both chancellor Philip Hammond and Brexit secretary David Davis in support of transitional arrangements.
But Downing Street has yet to clarify whether it will publish information after May's speech to further illuminate the Prime Minister's Brexit plan, and Fairbairn said that many businesses are still awaiting answers.
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"The longer that we don't have answers to key questions like, 'what is the new immigration model going to be?' and 'what is the access to our main trading partner going to be like?' business will be forced to make assumptions and that is why clarity matters so much," she said.
"The next three to six months will be vital for that."
Theresa May is expected to trigger Article 50, which will begin negotiations, in March.