Boris Johnson gestures as he takes questions during the launch of his bid to become the leader of the Conservative Party in London, Britain, June 12, 2019. (Photo: IC)

LONDON - Boris Johnson kicked off his campaign to succeed Prime Minister Theresa May on Wednesday with a pledge to lead Britain out of the European Union on Oct 31 and a warning to his divided Conservative Party that "delay means defeat".

Johnson, favourite for the top job nearly three years since he led the official campaign to leave the EU, praised the strength of the British economy, promised to deliver Brexit by Oct 31 and tackle despair across the country.

"After three years and two missed deadlines, we must leave the EU on October 31," he said as a heckler 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 and London mayor.

"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, whose unconventional style has helped him shrug off a series of scandals in the past, has won over many in 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.

When asked if he could be trusted, Johnson said he could.

Asked if he had ever broken the law, he spoke about driving above the speed limit. When asked about whether he had dabbled in illegal drugs, he dodged the question.

"Have I ever done anything illegal?...I cannot swear that I've always observed a top speed in this country of 70 miles an hour."

On drugs, he said: "I think what most people want us to focus on in this campaign, if I may say so, is what we can do for them and what our plans are for this great country of ours."