Jeremy Corbyn has told his fractious shadow cabinet “the debate is over” on Brexit, as he seeks to stamp his authority on the general election campaign and shift the focus to social justice and the climate emergency.

Speaking to the Guardian in the south-west London seat of Putney, the Labour leader claimed he had instructed frontbench colleagues to fall into line, after divisions over Brexit sparked a furious row over whether to go for an election.

“I just said, ‘look, this debate is now over. We’ve done it, the party has now made its decision, and that’s it; and that’s what we’re going to campaign on’,” he said, describing last week’s shadow cabinet meeting.

Speaking after a campaign visit to a social housing development, an uncharacteristically assertive Corbyn also said he had made a unilateral decision to back Boris Johnson’s plan to trigger a 12 December general election – despite the vocal objections of several colleagues, including Labour’s chief whip, Nick Brown.

“I put it to them quite clearly: I said, our objections are now gone. We are now supporting a general election – and everybody gulped. I didn’t alert anybody in advance – it was my decision. On my own. I made that decision. And they gulped, and said, Yes Jeremy.”

This general election is not about me, says Jeremy Corbyn – video

Labour’s stance on Brexit has shifted significantly over the past 12 months, and the party now says it would seek to negotiate a new deal with Brussels – including a closer trading relationship – within three months, and put it to the public in a referendum within six.

Several members of the shadow cabinet, including Keir Starmer, have suggested they would campaign for remain in that referendum, and Emily Thornberry recently described Labour as a “remain party”; but Corbyn has insisted any decision on Labour’s position will not be taken until after the election.

Corbyn rejected the idea that Jewish voters have anything to fear from a Labour government Photograph: Sean Smith/The Guardian

The detailed content of Labour’s manifesto will not be taken until the “clause V” meeting of senior Labour decision-makers, expected to take place next weekend.

But Corbyn insisted the Brexit policy decided at the party’s conference in Brighton, which saw a remain-supporting motion defeated by trade unions and constituency delegates, must stand.

“I don’t see why the clause V meeting would want to change anything, because my whole strategy has been to try and keep the party, the movement and the country together,” he said.

His robust stance echoed a piece written by union leaders including Unite general secretary Len McCluskey in the New Statesman, which called on shadow cabinet members to “keep their powder dry”.

If they throw personal stuff at me, it says more about them than me. They go low, we go high

The union leaders, including Dave Ward, general secretary of the Communication Workers Union (CWU) and Mick Whelan, general secretary of Aslef, alongside McCluskey, said, “Labour’s policy is scarcely going to maintain credibility if the very shadow cabinet members who would, in government, be responsible for negotiating a departure deal with the EU announce now that they will campaign for its public rejection. This is the politics of posturing, and not a serious approach to a pressing problem.”

The Lib Dem leader, Jo Swinson, has made stopping Brexit the centrepiece of her party’s pitch to the country. But Corbyn said Labour’s aim was to transform Britain.

“This election is about the future of this country. It’s about the environment. It’s about the cohesion of society. We can’t go on with austerity, poverty, inequality and injustice. We need a government that’s committed to reversing that. And we are the ones to do that.”

With veterans of the combative Vote Leave campaign running Boris Johnson’s bid for five more years in Downing Street, attacks on Corbyn’s character are likely to feature heavily.

But the Labour leader insisted he was undaunted. “I’ve had lots of mud thrown at me for a long time, and it doesn’t stick. I don’t care. I know what I believe, I know what we’re doing, I know what our policies are. If they throw personal stuff at me, it says more about them than me. They go low, we go high,” he said.

Asked whether he would attack Johnson’s character, he said: “I’m not doing personal. I don’t do personal. It doesn’t work.”

Corbyn meets Elias, Eesa and mother Sabiha during his visit to Clyde House, Putney, London. Photograph: Yui Mok/PA

Corbyn was speaking after a visit to Clyde House – a social housing block in Putney, where residents have fought a long-running grassroots campaign against their landlords, with the help of Labour community organisers.

Sitting on a sofa in a chronically overheated flat, he listened while tenants detailed their struggles to get broken lifts fixed and tackle damp, mould and shattered glass.

Labour has announced that it will spend up to £60bn upfront on retro-fitting existing homes with insulation and clean energy sources, in a “warm homes” pledge.

Corbyn said he believed the policy helped to show how Labour’s “green industrial revolution” could be a vote winner.

“My offer is: we will insulate your home, we will save you £400 a year on your bills, and we will create jobs, in fitting those homes, and manufacturing the sustainable energy sources,” he said.

And he insisted such policies would have appeal beyond allotment-tending north Londoners like himself, describing tackling the climate crisis as a “class issue”.

“It is a class issue – when it’s working-class communities that have the lowest life expectancy, the highest levels of pollution, the least efficient homes and the highest energy bills – whereas the wealthier middle-classes have cleaner air, longer lives, and much better insulated homes. And it’s the poorest children who eat the worst food, and have the greatest problems of obesity as a result of it,” he said.

Labour is kicking off the campaign significantly behind Johnson’s Conservatives in the national polls, and with the risk of the Lib Dems dividing the remain vote.

Corbyn interviewed by Heather Stewart. Photograph: Sean Smith/The Guardian

The Tories are banking on winning a string of Labour-held seats, particularly in Brexit-voting areas, with a tough stance on law and order, and by painting Corbyn as a threat to national security.

Asked whether he could promise voters that he would protect them and their families, the Labour leader said: “What is real security in your life? Real security is somewhere to live, real security is a secure job, real security is a health service, real security is social care, real security is knowing that you can bring up your children in a safe, clean, sustainable environment.”

Pressed on whether he was prepared for the “3am call,” waking a prime minister to warn Britain is under imminent threat, he said: “Of course: absolutely. You have to take decisions at times like that. The principle behind it will be sustainable peace, will be human rights, will be justice.”

Corbyn also rejected the idea that Jewish voters have anything to fear from a Labour government, after the Jewish Labour Movement said it would barely campaign for the party during the general election, amid concerns over its handling of reports of antisemitism among members and supporters.