UPDATE 6-With Oct. 31 Brexit pledge, Boris Johnson bids for UK leadership

* Johnson kicks off bid to be British PM

* Johnson: We must leave EU on Oct. 31

* Vows to unite country split over Brexit

* Dodges questions on drugs and crime

* Rival says Johnson is "yesterday's news" (Adds Javid, parliament vote)

By Elizabeth Piper and William James

LONDON, June 12 (Reuters) - Boris Johnson kicked off his campaign to succeed Prime Minister Theresa May on Wednesday with a commitment to lead Britain out of the European Union on Oct. 31, warning his divided Conservative Party "delay means defeat".

Johnson, favourite for the top job nearly three years since he led the referendum campaign to leave the EU, praised the strength of the British economy and promised to tackle widespread disillusionment over the protracted Brexit process.

To a packed room where some supportive lawmakers were forced to stand, Johnson drew on his past as a former London mayor to try to persuade Conservatives that only he could take the party to election victory, explaining away some of his well-documented gaffes as his desire to "speak as directly as I can".

"After three years and two missed deadlines, we must leave the EU on October 31," he said as a heckler repeatedly yelled "Bollocks to Boris" from outside the Royal Academy of Engineering, just off The Mall in central London.

"I am not aiming for a no-deal outcome," said Johnson, a 54-year-old former foreign minister.

"I don't think that we will end up with any such thing, but it is only responsible to prepare vigorously and seriously for no-deal. Indeed it is astonishing that anyone could suggest dispensing with that vital tool in the negotiation."

Johnson is one of 10 contenders to replace May, whose premiership crumbled after she repeatedly failed to convince parliament to ratify the divorce deal she negotiated in November with the EU.

After Johnson was widely praised by supporters, one rival, Rory Stewart, said Johnson could be his own worst enemy during a campaign that will run until late July. "On reflection I am beginning to think there are only two candidates who can beat Boris - me, and Boris himself," he said on Twitter.

HIS TO LOSE

Johnson, whose unconventional style has helped him shrug off a series of scandals in the past, has won over much of his party by arguing that only he can rescue the Conservatives by delivering Brexit.

For many, the contest for prime minister is his to lose - he has the most declared Conservative supporters in parliament and is widely popular among the party's members, the people who will ultimately choose May's successor.