(Changes date, adds EU position)
By Elizabeth Piper
LONDON, June 12 (Reuters) - Boris Johnson, favourite to succeed Prime Minister Theresa May, will commit on Wednesday to taking Britain out of the European Union on Oct. 31, warning the governing Conservatives that "delay means defeat" at a campaign launch.
In a crowded race to replace May, Brexit has become the touchstone policy for the contenders, with Johnson, who led the campaign to leave the EU almost three years ago, taking a tough stance to exit at the end of October, with or without a deal.
The former foreign secretary and ex-London mayor has won over many in his party by arguing only he can rescue the Conservative Party, which has been punished in recent elections over the government's inability to deliver Brexit on time.
And 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.
As in the 2016 referendum on EU membership, Johnson's message is clear - any more Brexit delays and the Conservative Party risks opening the door to a government led by opposition Labour leader and veteran socialist Jeremy Corbyn.
"After three years and two missed deadlines, we must leave the EU on October 31," he will say, according to extracts from his speech.
"We simply will not get a result if we give the slightest hint that we want to go on kicking the can down the road with yet more delay. Delay means defeat. Delay means Corbyn. Kick the can and we kick the bucket."
Almost three years since voting to leave the EU, Britain is no clearer on how, when or even whether Brexit will happen. Parliament has rejected the divorce deal negotiated with the EU three times and there is little agreement among lawmakers on Brexit.
Johnson hopes to win over Brexit supporters by promising to leave on Oct. 31 deal or no deal - a pitch that might win him support in his leadership bid, but one which also worries pro-EU lawmakers who have launched an attempt to stop any future government from leaving without a deal.
The EU has refused to renegotiate the Withdrawal Agreement reached with May last November, and Ireland has indicated it is not willing to change the Irish border "backstop" that upset the party which props up May's minority government.
Johnson has been criticised for hiding "in a bunker" by some of his opponents, but the strategy to reduce the media exposure of a man who has been prone to gaffes and scandals seems to be working, with his support so far holding up.
For many in the Conservatives, he is seen as one of a few prime ministerial hopefuls who could lead the party to another election victory after nine years in power.
"We cannot let them (Corbyn's Labour) anywhere near Downing Street and I would remind you that the last time I faced an emanation of that Marxist cabal I defeated him when the Conservatives were 17 points behind in London," he will say on Wednesday, referring to his victory as the capital's mayor in 2008.
"And we can do it again."
(Editing by Janet Lawrence and Andrew Heavens)
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