British businesses urged Prime Minister Theresa May on Monday to provide clarity on how Brexit will work, but she gave few details beyond reiterating plans to seek a transition deal as soon as possible.

Britain is due to leave the EU, its largest trading partner, in March 2019, but negotiations on how goods and services will flow between the two after that date have made slow progress, fueling fears of an exodus of multinational firms.

In a speech to Britain's most powerful official business lobby group, May said she understood how important clarity was for business and that she hoped for what she calls an "implementation period" that would allow the same trading conditions to continue for around two years after the exit date.

But the president of the Confederation of British Industry chided the government, saying its Brexit strategy reminded him of a soap opera with a different episode each week and that Britain was less united than the EU's 27 other states.

"Brexit is only 508 days away, but for many businesses their alarm clocks are set earlier than that: They're set to the moment they'll actually enact their contingency plans," CBI President Paul Drechsler said just before May spoke. "The clock is ticking, so government and the EU need to get a move on... First of all sorting out, with clarity, details on a transitional arrangement."

BT, Britain's biggest telecoms company, said that unless businesses were clear by the beginning of 2018 about the terms of a transitional period they would need to plan for a damaging "hard Brexit".

Ever since the June 2016 EU referendum, business leaders have been calling on the government to give them clarity about how Brexit will look, though ministers say that is impossible, given that the exit terms have still to be negotiated.

"I know how important it is for business and industry not to face a cliff-edge and to have the time it needs to plan and prepare for the new arrangements," May told the CBI conference.

"I want us to agree the detailed arrangements for this period as early as possible."

The CBI's director general, Carolyn Fairbairn, said that the government's Brexit ambitions must be matched with "urgent delivery".