Ten candidates have been nominated in the contest to succeed Theresa May as Britain's Conservative Party leader and prime minister.
The candidates include frontrunner former foreign minister Boris Johnson, his successor Jeremy Hunt, environment minister Michael Gove, former Brexit minister Dominic Raab, health minister Matt Hancock and interior minister Sajid Javid, the Conservative Party's 1922 Committee said.
The other candidates are international development minister Rory Stewart, former minister Esther McVey, former leader of the House of Commons Andrea Leadsom and Conservative MP Mark Harper.
The first round of voting among Conservative MPs to begin whittling down the field to two candidates, who will then be put to a vote of party members, will take place on Thursday.
Mrs May stepped down as leader of the 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.
An 11th candidate to replace her, Sam Gyimah, withdrew shortly before the announcement saying he had not been able to build sufficient support. He was the only one to support holding a second Brexit referendum.
All the public campaign launches on Monday set out domestic agendas, but it was Brexit that dominated, with overt and thinly-veiled digs at former foreign minister Boris Johnson.
"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'," said environment minister Michael Gove, who scuppered Mr Johnson's 2016 leadership bid by pulling his support at the last moment to run himself.
"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'."
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 31 October.
"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," current foreign minister Jeremy Hunt said.
He warned that, unless the issue was sorted out, the party would be annihilated in an election and socialist Labour leader Jeremy Corbyn would take power.
'The Brexiteer you can rely on'
Dominic Raab, who quit as Brexit minister over Mrs May's divorce deal, said he too could secure a new agreement but promised that the United Kingdom would leave the EU on 31 October, even if that meant reverting to basic World Trade Organisation trade terms.
"I'm the Brexiteer that you can rely on," he said.
Others, including Mr Johnson, have made the same promise to leave on time even if it means giving up on a deal with the EU to smooth the transition.
The differences between the candidates reflect Conservative disunity on the issue, which has meant that, three years after the United Kingdom voted by 52 percent to 48 percent to quit the EU, it remains unclear how, when or even whether it will indeed leave.
The uncertainty has hit Britain's economy, which shrank by 0.4 percent in April, official figures showed on Monday - a bigger drop than any economist had forecast in a Reuters poll last week.
Boris Johnson is not only the bookmakers' clear favourite but, according to polls, the most popular with the 160,000 party members who will ultimately make the choice.
He did not hold a high-profile campaign launch on Monday, but his rivals lined up to take potshots at him and his pledge to raise the point at which workers begin paying a 40 percent income tax to £80,000 from £50,000.
"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," said Gove, another frontrunner who was trying to get his own campaign back on track after admitting taking cocaine when he was a young journalist.
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.
'We need a serious leader'
Others took aim the flamboyant former London mayor, alluding to past criticism that he favoured style over substance and failed to grasp details.
"We won't get a good deal with bluff and bluster," said Raab, one of the most hardline Brexit advocates.
Mr Hunt, another of the favourites, said "a serious moment calls for a serious leader". "We need tough negotiations," he said, "not empty rhetoric".
Mr Hunt's own campaign received a significant boost on Monday when two ministers with opposing views on Brexit - pensions minister Amber Rudd and defence minister Penny Mordaunt - endorsed him.
While the battle unfolds, Mrs May remains prime minister. Her replacement is due to be in place by the end of July.
- Reuters / BBC