British Prime Minister Theresa May has won the backing of two senior ministers as she battled to defend her draft Brexit deal, while calls grew for a vote of no confidence in her leadership.

More than two years after the United Kingdom voted to leave the European Union, it is still unclear how, on what terms or even if it will leave the EU as planned on March 29, 2019.

Mrs May, who won the top job in the turmoil that followed the 2016 referendum, announced she had agreed a Brexit deal which ensured the United Kingdom left the world’s biggest trading bloc in the smoothest way possible. But Brexit minister Dominic Raab resigned over her deal, sending the pound tumbling, and he was followed by a number of other colleagues.

Mrs May, who has defiantly vowed to stay on as prime minister, spoke on a radio phone-in show to defend her deal, and was asked by a caller to “respectfully stand down”.

She did not immediately address that part of the caller’s question. Influential Conservative MP Steve Baker said rebels in Mrs May’s party were close to the threshold that would trigger a no-confidence vote.

Such a vote must take place when 48 Conservatives submit letters to the party’s so-called 1922 committee.

“What’s happened is people have been ringing me and telling me that they are putting their letters in … and I think we’re probably not far off,” said Mr Baker, a key figure in the Brexit-backing wing of Mrs May’s party. “I think it probably is imminent, yes,” he told BBC TV.

However in a boost to Mrs May, Michael Gove, 51, the most prominent Brexit-supporting minister, gave his backing to her, saying he would stay on in government as environment minister.

Asked if he had confidence in Mrs May, Mr Gove, who torpedoed former foreign minister Boris Johnson’s leadership bid in 2016, told reporters: “I absolutely do”.

“I think it’s absolutely vital that we focus on getting the right deal in the future, and making sure that in the areas that matter so much to the British people we can get a good outcome,” he said.

Trade minister Liam Fox, another leading Brexit supporter, also backed Mrs May, while her de facto deputy David Lidington said she would win a vote of no confidence.

Meanwhile, Mrs May has filled two vacancies in her Cabinet left by ministers who quit over Brexit.

Little-known politician Stephen Barclay was named Brexit secretary after Mr Gove reportedly turned down what is viewed as a poisoned chalice.

Amber Rudd was made work and pensions secretary, replacing Esther McVey.

Ms Rudd is a former Cabinet minister who resigned as home secretary in April amid a scandal over authorities’ mistreatment of long-term UK residents wrongly caught up in a government drive to reduce illegal immigration.

She is a longstanding ally of Mrs May’s and has said she supports the proposed divorce agreement with the EU.

Sterling, which has seesawed on Brexit news since the referendum, was up half a cent against the US dollar at $US1.28 ($A1.77) on Friday.

To leave the EU on the terms of her deal, Mrs May would need to get the backing of about 320 of parliament’s 650 MPs.

The deal is due to be discussed at an EU summit on November 25.