She will remain as caretaker prime minister until her successor is chosen.
Mr Javid's background as the son of Pakistani immigrants sets him apart from numerous other Conservative contenders.
The last minister to quit May's Cabinet before the premier announced her own resignation, Leadsom said Brexit must be delivered by the October 31 deadline. And the list is long, with 10 candidates in total.
"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", Gove said.
A series of votes are expected to whittle down the number of candidates to two, and the new leader will be announced by the party in the week starting July 22.
Among some of the others who formally launched their leadership bids on Monday include United Kingdom health secretary Matt Hancock, who called for a "fresh start", and former Brexit Secretary Dominic Raab, who unveiled proposals to redirect GBP 500 million a year from the UK's aid budget to create an worldwide wildlife fund to save endangered species and habitats. "The field remains open for that elusive leader".
Boris Johnson leaves his home in London.
Britain is set to get a new prime minister, but only members of the Conservative Party have a say in who will next lead the country.
"We won't deliver Brexit with bluff and bluster", said former Brexit Secretary Dominic Raab, who is competing with Johnson for the support of hardcore Brexiteers.
Foreign Secretary Jeremy Hunt said a "very smart" approach was needed for Brexit talks, saying an "experienced, serious leader" was needed, not "empty rhetoric".
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Raab, the former Brexit secretary, is another hard Brexiteer who was accused by the EU's Selmayr of "pure disinformation" and of making "fraudulent" claims on the future of Ireland after Brexit.
Now the party and the country may have a true Brexiteer at the helm.
The differences between the candidates reflect Conservative disunity on the issue, which has meant that, three years after the United Kingdom voted by 52percent to 48 percent to quit the European Union, it remains unclear how, when or even whether it will indeed leave.
"Without Brexit, there will be no Conservative government and maybe no Conservative party", foreign minister Jeremy Hunt said at his campaign launch.
Hunt has aimed to pitch himself as a moderate who will accommodate Remainer concerns while delivering Brexit.
However, he could find, as May did, that trying to appease both sides eventually makes everyone unhappy.
Hunt's campaign is also supported by Amber Rudd, another Theresa May loyalist.
Gove, the environment minister who was tipped among the frontrunners to be the UK's next prime minister, acknowledged over the weekend to having used drugs, prompting a handful of scathing rebukes in the media and from politicians.
"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.
His recent admission that he took cocaine while he was working as a journalist has damaged in his standing as a frontrunner.