Mr Umunna also did not rule out joining the euro if the UK rejoined and set out a vision of the continent with an “economic core” based around Germany and a “defence and security core” led by Britain.
His comments, in an on-stage conversation with Independent editor Christian Broughton, came ahead of his first keynote conference speech as a Liberal Democrat MP, in which he will brand Boris Johnson a “peddler of hate” whose populist politics threaten “the Britain we know and love”.
The prime minister stands on the wrong side of a “new fault line” in modern politics dividing liberal internationalists from authoritarian nationalists, he will say.
And he will reject Labour’s claim to be on the side of liberal values of liberty, equality and community, saying that neither Jeremy Corbyn nor Mr Johnson are fit to lead the country.
The Bournemouth conference voted overwhelmingly on Sunday to revoke the Article 50 Brexit process without a referendum if they win power.
Asked shortly after the vote whether he would campaign for British re-entry to the EU if Brexit goes ahead, Mr Umunna answered with an unequivocal: “Yes.”
But he added: “I don’t accept that we’re going to leave the European Union.”
Pressed on whether he would be ready for the UK, as an applicant member, to sign up to the euro and any putative European army, he said: “Look, I don’t think that is something that will be demanded of us, not least because about a third of members of the European Union are not part of the euro, and I do not see them becoming part of the euro.
“This idea of a European Army I think is unlikely to come about, in the configuration that is suggested by the red-top tabloid newspapers.
“One of the tragedies of the trajectory we are on of potentially leave the EU is of course the EU has to reform. I always thought that going forward you would have an economic core which would revolve around Germany – the main economic powerhouse in the eurozone – and then it made sense to have a defence and security core that Britain would lead.”
In his speech today, Mr Umunna will say that his own mixed English, Irish and Nigerian heritage is an example of the “liberal, open, internationalist spirit” which Lib Dems cherish.
He will say that his new party “exists to build and defend a fair, free and open society” and to fight for the rights of those from all backgrounds to live together in peace.
In a swipe at Mr Johnson’s suggestion that he might not obey anti-no-deal legislation and Tory jibes at judges, Mr Umunna will say that Lib Dems demand that governments “respect the rule of law with an independent judiciary ... free from abuse and attack by the executive”.
“This is the Britain we know and love, and Boris Johnson, Nigel Farage and the other peddlers of hate and division had better know that this is what we will fight for at the coming election,” he will say.
“It is our job to make sure this country’s heartbeat is liberal and internationalist, not nationalist, populist and authoritarian. It is the new fault line in British politics and we know where we stand.”
— Liberal Democrats (@LibDems)September 15, 2019
Mr Umunna will say that, under Mr Corbyn, his old party Labour “is not the party of Attlee and Bevin”.
Labour cannot claim to be a “champion of liberalism” so long as it is under investigation over antisemitism and when activists feel able to “abuse, vilify and deselect” critics of the leader, he will say.
And he will accuse Mr Corbyn of being an “apologist” for Russia’s Vladimir Putin and lauding authoritarian regimes in Iran and Venezuela while seeking to abolish Nato.
“What unites both Johnson and Corbyn is the fact that both want to leave the EU, this biggest champion of liberalism in our neighbourhood,” he will say.
“Neither is fit to lead this country. It’s time for a change and someone I know can provide that leadership – Jo Swinson.”
Elsewhere at the Bournemouth conference, Lib Dem Treasury spokesperson Sir Ed Davey will commit the party to introducing a “wellbeing budget” to help the most vulnerable in society, as well as a “climate emergency budget” to support the drive to decarbonise capitalism.