May's successor will face the same hung Parliament, and Leadsom said she'd work with Labour backbenchers - not the leadership - to shape post-Brexit ties to the European Union in a way that can win the support of the House of Commons.
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. In a series of secret ballots running from June 13 to 20, MPs will then whittle down the field to a final pair.
Mr. Johnson is expected to launch his campaign on Tuesday and so far he has kept a lower profile than the other candidates. While all were keen to set out a domestic agenda, it was Brexit that dominated.
Hunt opposed Brexit in the 2016 European Union referendum but said that the Conservatives - under pressure from populist euroskeptic Nigel Farage's new party - must deliver on the result or face "annihilation" in the next election.
Johnson is promising to lead Britain out of the European Union by October 31, with or without a divorce deal.
"The danger is that if we vote for a potential prime minister who said whatever happens I'm doing to leave to leave on October 31, we will be voting for a prime minister who will not be able to deliver Brexit before there's a general election", he said. It "it "probably tells you that I was doing my job in terms of pressing them hard and making sure that Britain's interests were resolutely defended", he said".
In endorsing Hunt, Rudd said: "These are serious times and we need a respected statesman who Brussels will listen to, not more bluster".
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.
The front-runner is former foreign secretary Boris Johnson, a fierce critic of Ms.
Ms Patel, a former United Kingdom global development minister, said that she believes Mr Johnson is also the right candidate to strengthen Britain's bond with countries like India.
The move would cost 9.6 billion pounds a year.
However, he looks determined to carry on with his bid saying he is "undaunted" by criticism because he believes he can deliver Brexit.
Foreign minister Hunt, another frontrunner, said "a serious moment calls for a serious leader". The new leader will be announced during the week of July 22 and he or she will take over as prime minister from Ms. May.
It isn't the first clash between the candidates, with Michael Gove hitting out at Boris Johnson during his campaign launch yesterday.
The 51-year-old's campaign this time has been hit by the revelation in a forthcoming biography of his illegal use of cocaine, which he admitted but said was a mistake.
"We simply cannot make spending and tax cut promises that we can't keep", he said.
The person chosen as leader will automatically become Prime Minister, though Queen Elizabeth II has to approve the appointment.
"We need someone who has been tested in the heat of battle. not someone who will hide in their bunker", said Environment Secretary Michael Gove, who had directly criticised Johnson for his newly-announced proposal of cutting taxes for the rich. Gove had made a decision to run for premiership three years ago and this put a dampener on Johnson's chances then.
Her intervention comes as Tory lawmakers on either side of the Brexit divide begin to pick the candidates they are backing in the prime ministerial race, which remains wide open with almost a dozen hopefuls in the fray.