Jeremy Corbyn's election as leader has strengthened Labour's campaign to keep Britain in the European Union, Alan Johnson has said.
The former Cabinet minister insisted the surge of new party members would allow the party to reach parts of the country which otherwise "would have been difficult".
Mr Johnson, who is leading Labour's campaign to stay in the EU, added it would be "perverse" to think of any circumstances which would result in Brexit.
The Labour MP for Hull West and Hessle said Mr Corbyn also wants Britain to stay in the European Union "no matter what" emerges from the Prime Minister's negotiations.
Mr Corbyn has indicated his party will support Britain's continued membership of the EU during the in/out referendum promised by David Cameron by the end of 2017.
He noted in the Financial Times that the shadow cabinet opposes reforms being sought by Mr Cameron that would reduce workers' rights but agreed the answer was "not to leave the EU but to pledge to reverse those changes with a Labour government elected in 2020".
Speaking after a fringe event at Labour's conference in Brighton, Mr Johnson told the Press Association: "I met Jeremy last week. He is absolutely 100% in favour of this campaign.
"His argument is no matter what emerges from the Cameron negotiations we have to stay in Europe in order to determine our future.
"I actually think with all the huge influx of new members we can reach parts of the country now that would have been difficult without this.
"We've now got the biggest party, we are the pro-European party who can deliver, unlike the Liberal Democrats who have shrunk away, the Tories who are divided and Ukip who are opposed.
"So I think we're in a pole position, and it's actually a stronger position post-Jeremy Corbyn's election."
On what needs to be done for the pro-EU campaign to succeed, Mr Johnson said: "Get the campaign going, there's no point waiting. It could be any time between April and October next year, probably around this time next year.
"We can't wait for special conference or further debates. We know where we stand as a party. It's perverse to think that there would be any circumstances in which we would come out."