An 11th, Sam Gyimah, withdrew shortly before the announcement saying he had not been able to build sufficient support.
May resigned as Conservative leader last week after failing three times to secure Parliament's backing for her divorce deal with the EU.
Numerous candidates have been campaigning unofficially for weeks, laying out policies on everything from policing to tax, and facing questions about past drug use.
The contentious issue of the backstop has been one of the major stumbling blocks in the Brexit negotiations. He had however said that the Conservatives under pressure from populist eurosceptic Nigel Farage's Brexit party must live up to expectations of voters or face "annihilation" in the next election.
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.
Another EU source told The Guardian: "The Tory party will be in survival mode and will have to regain credibility as the party of Brexit". While Johnson remains the clear favourite to win the race, he also was in 2016 when he was forced to step down, opening the way for then fringe candidate Theresa May to become leader.
Mr. Johnson's main rival in the race was supposed to be Environment Secretary Michael Gove, 51.
Michael Gove's leadership prospects have been marred by his recent admission that he had taken cocaine "on several occasions" while a journalist in the 1990s.
Mr Javid's background as the son of Pakistani immigrants sets him apart from numerous other Conservative contenders.
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The contest's victor will be chosen in a two-stage process.
"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.
The final two will be put to the 120,000 or so members of the wider Conservative Party in a vote from 22 June, with the victor expected to be announced about four weeks later.
The favourite on betting markets is Johnson, a former Foreign Secretary with an instantly recognisable mop of blond hair and a knack for entertaining the public.
Most have given television and radio interviews and held public launch events - things Johnson has so far avoided, in an attempt to reduce the chance of gaffes that could derail his campaign.
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".
Hunt said the party needed a "serious leader".
The 52-year-old says he will push hard for a new deal with Brussels without taking the possibility of a no-deal outcome off the table.
Gove has signalled he could be open to delaying Brexit again rather than leave without a deal on October 31.
"There would be a vote of confidence in the House of Commons that the Government would lose", Gove said.
Eleven MPs are vying to replace May and whoever succeeds her as head of the party will automatically enter Downing Street.