LONDON (REUTERS) - Leading Brexit advocate Boris Johnson promised tax cuts for higher earners if he becomes Britain's next prime minister as the contest to replace Theresa May - and take charge of the EU divorce - officially kicked off on Monday (June 10).

May stepped down as leader of the ruling Conservative Party on Friday, having failed three times to win Parliament's support for a European Union divorce deal that was supposed to address Britain's biggest political crisis in a generation.

Nominations to replace her must be submitted on Monday, and each of the 11 declared candidates needs to secure at least eight backers from the Conservatives' 300-plus elected lawmakers.

A number of the contenders look set to fall short, and voting later this week will whittle the field down further.

At campaign launches on Monday, several made their pitches. While all were keen to set out a domestic agenda, it was Brexit that dominated.

Nearly all promised that they could solve the Brexit conundrum - which eluded May in three years of EU talks - in just three months, between the new leader being chosen at the end of July and the current exit date of Oct 31.

"Without Brexit, there will be no Conservative government and maybe no Conservative party," foreign minister Jeremy Hunt said at his campaign launch. "From my conversations with European leaders, it is clear to me there is a deal to be done; they want us to come up with proposals."

"THE BREXITEER YOU CAN RELY ON"

Dominic Raab, who quit as Brexit minister over May's divorce deal, said he too could get a new agreement but promised that the United Kingdom would leave the EU on Oct 31, even if that meant reverting to basic World Trade Organisation trade terms. "I'm the Brexiteer that you can rely on," he said.

At his launch, health minister Matt Hancock, who has ruled out a no-deal departure, said: "We don't need a 'leaver', we don't need a 'remainer', we need a leader for the future."

The differences between the candidates reflect the Conservative disunity on the issue, which has meant that, three years after the United Kingdom voted by 52 per cent to 48 per cent to quit the EU, it remains unclear how, when or even whether it will leave.

The uncertainty has hit Britain's economy, which shrank by 0.4 per cent in April, official figures showed on Monday - a bigger drop than any economist had forecast in a Reuters poll last week.

Former foreign minister Johnson is the bookmakers' clear favourite to succeed May and, according to polls, the most popular with the 160,000 party members who will ultimately make the choice.

He did not launch his campaign with a high-profile appearance but instead outlined his plans in a newspaper column and interview.

However, his pledge to raise the point at which workers begin paying a 40 per cent income tax to 80,000 pounds (S$138,656) from 50,000 still attracted much media attention, and prompted rivals to say they would rather focus on helping the lower-paid.

The move would cost 9.6 billion pounds a year.

"WE NEED A SERIOUS LEADER"

Johnson, who has also suggested that London could withhold an agreed 39 billion pound Brexit payment, is one of a number of candidates who has promised to take Britain out of the EU with or without a deal on Oct 31.

The flamboyant former mayor of London has been accused in the past of focusing on style over substance and failing to grasp details, something to which his rivals alluded.

"We won't get a good deal with bluff and bluster," said Raab, one of the most hardline Brexit advocates.

Foreign minister Hunt, another frontrunner, said "a serious moment calls for a serious leader". "We need tough negotiations," he said, "not empty rhetoric."

Hunt's campaign received a significant boost when pensions minister Amber Rudd and defence minister Penny Mordaunt gave him their backing.

Environment minister Michael Gove, who was also considered a frontrunner, was hoping to get his campaign back on track after admitting taking cocaine when he was a young journalist.

Gove apologised but critics accused him of hypocrisy, noting that, in a previous role as education minister, he had signed off on rules to ban teachers for life for taking cocaine.

Taking a dig at Johnson's campaign, Gove said on Monday he would never use the taxation system to give the wealthy a tax cut. "One thing I will never do as prime minister is to use our tax and benefits system to give the already wealthy another tax cut," Gove said in a speech to launch his campaign.

Gove said Britain needed a serious leader and quipped: "If I get through, which I am sure I will actually, to the final two against Mr Johnson, this is what I will say to him: 'Mr Johnson, whatever you do, don't pull out, I know you have before, and I know you may not believe in your heart that you can do it, but the Conservative Party membership deserve a choice'."

While the leadership battle unfolds, May remains prime minister. Her replacement is due to be in place by the end of July.