British Prime Minister Theresa May on Monday won support from big businesses for her draft Brexit deal, but ally the Democratic Unionist Party (DUP) issued a warning by refusing to back the government in a series of finance votes.

May told the Confederation of British Industry (CBI), the UK's main business lobby group, that she was "determined to deliver" her Brexit deal as she prepares for Sunday's European Council summit to sign Britain's divorce papers with the EU.

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"We now have an intense week of negotiations ahead of us," she told some 1,000 business leaders at the CBI annual conference in London.

"I expect us to hammer out the full and final details of the framework that will underpin our future relationship. I am confident that we can strike a deal at the Council that I can take back to the House of Commons."

The embattled premier, whose cabinet grudgingly approved her EU divorce draft last week, faces an uphill challenge to pass her deal through a skeptical parliament. She admitted that the final phase "was always going to be the toughest."

As well as pro-EU lawmakers, she is struggling to convince many pro-Brexit lawmakers in her own Conservative Party and the Northern Ireland's Democratic Unionist Party, which props up her government in parliament.

DUP lawmakers on Monday failed to back the government on several votes, a sign of the struggle May faces in trying to get her plan through a parliament, which like much of Britain, is still deeply split over Brexit.

CBI chief Carolyn Fairbairn gave qualified support for May's draft deal, meanwhile, telling the conference that "it is not perfect, it is a compromise, but it is hard-won progress."

She earlier said that the deal would move Britain "one step away from the nightmare precipice of no deal."

"It offers that essential transitional period as a step back from the cliff-edge," she said.

Responding to reports that EU chief Brexit negotiator Michel Barnier had raised the prospect of extending the transition deal until the end of 2022, May said it was "important that we are out of the implementation period by the next general election," scheduled for May 2022.

Addressing the issue of immigration, a concern for many businesses who fear they will face skills gaps after Brexit, May promised to introduce "more streamlined application processes" that will "attract the brightest and the best from around the world."

Jeremy Corbyn, leader of the main opposition Labour Party, said his MPs would vote against the draft deal then demand a general election if lawmakers reject the plan.

The British government also on Monday pledged to publish economic forecasts that compare May's Brexit deal with remaining in the European Union, relenting to pressure from lawmakers who sought to force the government to do so.

More than 70 lawmakers, including several in May's governing Conservatives, had supported the so-called amendment to the finance bill, to demand the government publish the analysis before a vote on the deal comes before parliament.