Trade unions have not watered down Labour’s manifesto pledges on climate and migration, Unite’s Len McCluskey has insisted. In a wide-ranging interview with the New Statesman, McCluskey — the general secretary of Labour’s most powerful union – said the manifesto, due to be launched on Thursday, would contain an “absolute commitment” to a 2030 target for net-zero carbon emissions.

Reports have suggested that the final text of the manifesto states that a Labour government would make “substantial” progress towards net-zero by 2030, as opposed to the “overwhelming” progress proposed in the original draft, after lobbying from the GMB union.

On immigration, Labour has also agreed a compromise position that falls short of its conference commitment to “maintain and expand free movement”. McCluskey had said in advance of Saturday’s Clause V meeting that the conference policy was not a “sensible approach”.

Asked to respond to claims that union interventions had resulted in a less radical policy programme than the party’s grassroots would like, McCluskey told the NS: “I don’t think that’s true at all… I think that people will see that it is a radical programme, certainly in relation to climate change. Labour is the only party that is seriously engaging in a way to bring about real change in the climate.

“The 2030 target will be in there, with a real commitment. For example, with my own union, there’s been lots of debate and discussion with what we do with our energy companies, how we develop green energy and sustainable alternatives. So I don’t think people will be displeased with that.”

Stressing that Unite supported the Green New Deal policy, as passed by Labour conference in September – which requires Labour to “work towards a path to net-zero carbon emissions by 2030” – he added: “There were concerns raised by the GMB – perfectly legitimate concerns – but that was talked about, and we reached a decision on that. My union is committed.”

On migration, he added: “I’m certainly not opposed to the free movement of labour, but obviously what we are saying is that that labour, wherever it’s coming from, has to be protected against the ravages of greedy, greedy bosses.

“They see migrant labour as vulnerable workers who they can abuse, and Labour is the only party that will step in with labour market regulations to protect them, and in that way, any concerns about undercutting of wages are dealt with. It’s only Labour that will do that. Hopefully, people will see that as a very radical step.

“I certainly see it as a correct and principled and humane way of dealing with the issues that are currently being talked about. This idea that somehow against migrant labour, or against free movement is completely and utterly wrong. It’s the opposite. It’s protecting migrant labour from being abused.”

Though McCluskey’s interventions in the election debate have been repeatedly cited by Jo Swinson, the Liberal Democrat leader, as evidence that Labour has become a cold house for Remianers, he argued that pro-EU voters would “flood” to support the party’s policy on a second referendum.

Appealing to Remain voters to support Jeremy Corbyn, he said: “Everybody who voted to Remain and feels that powerfully about it should vote Labour.

“Labour is the only party that is offering the chance of a second vote on this. I think the Liberal Democrats have made a huge, huge error, and I can see them being severely punished for it, unless in some Tory seats Labour voters decide to vote tactically where the Liberal Democrats are second.

“But for Jo Swinson to adopt a position of revoking Article 50, and whilst she’s doing it, revoking a democratic result where 17.4 million people voted to Leave, is extraordinary. I think she’s shot herself badly in the foot. That’s the most illiberal position that you could take… Labour’s position is honourable and credible and principled. I believe Remainers will flood to it.”

He added: “My message to Liberal Democrats is: Labour is the only party that offers you a way to unite our nation. My message to voters who support the Green Party: it’s Labour you should be giving your vote to, because it’s only Labour who can deliver on a green agenda that people who are committed to the Green Party obviously want.”

McCluskey – who joined three other union leaders in criticising shadow cabinet ministers for declaring they would vote Remain in an NS piece earlier this month – stressed that Labour’s "bigger task" was to "persuade Leave voters, certainly in our heartlands” to stick with the party and “come home to Labour.”

“People voted to Leave for maybe two main reasons,” he said. “One: the forgotten towns and cities being deindustrialised. But it’s only Labour that has policies to do something about that. The second reason was the issue and concerns about migrant labour in particular. Again, it’s only Labour that has policies to deal with those concerns.

“Leave voters who are still feeling undecided about whether they should vote Conservative or for the Brexit Party need to understand that the Conservatives will do nothing about those two main concerns. They have no policies to invest in communities, to invest in real manufacturing jobs in those forgotten towns and areas. In fact, it’s their policies that led to the deindustrialisation, and they’ve got no plans to reverse that.”

“My message, and I’m going to be going around to some of our heartlands, is stay with Labour, come home to Labour. Don’t give your vote to a bunch of Tories who don’t understand you. Don’t give your vote to the phonies of the Brexit Party. Stay with Labour.”

Speaking ahead of this evening’s head-to-head TV debate between Corbyn and Boris Johnson, McCluskey argued that the launch of Labour’s manifesto offered the party a chance to convince voters that “there’s more to life than Brexit”.

“Let’s talk about all of those issues that are so important to people: a British investment bank, regional investment banks, investing in our infrastructure and manufacturing, eliminating zero-hour contracts, a £10 minimum wage, protecting the NHS and our public services – which everybody wants more of!

“Committing to build a million homes over a decade. Tackling homelessness. Lifting the student debt off our young people’s shoulders. All of these things Labour is promising. The deal that they’re going to put to people is unbelievable. Free prescriptions. Free parking in hospitals, I mean, that might sound small beer, but do you know what? I know, and you’ll know, and everyone reading your thing will know hundreds, thousands of people who’ll be infuriated that they get fleeced every time they go and visit their loved ones who are sick.

“All of these things add up to an unbelievable offer that Labour is going to be making, one that will transform our society.”

McCluskey went on to say: “When you look at Labour’s programme – oh my god. You would think that we would end up with a massive landslide, because it speaks to so many people.”

Asked why current polling did not suggest an overwhelming Labour victory, he admitted that the Brexit debate had made it more difficult for Labour’s domestic agenda to cut through.

Accusing the Conservatives of lying about the cost of Labour policies, he said: “There is no doubt that Brexit has poisoned the political debate. It has been so polarised. It’s why our nation is split in two. You look at Johnson, who peddles that division. You look at Jo Swinson, who peddles that division. It is incredibly difficult for a leader like Corbyn to try to unite.”

He nonetheless added: “I’m an optimist, and I know that the message we are taking forward is a strong one, is a powerful one, and the challenge is to get it home.”

Describing the prospects of a Conservative victory as a “bad, bad thing” for the British economy and workers' rights, McCluskey said a “period of reflection” would be required in the event of a Labour defeat.

Pressed on whether Corbyn would have to resign as leader, he said: “Well, obviously I’m not expecting Jeremy to lose. Jeremy would have to make a decision, along with other sections of the party. We would have to look at things – that’s why I say a period of reflection. Back in 2015, Ed Miliband resigned the following day. I think that was wrong of him to do that. You always need a period of reflection.

“The one thing I know about Jeremy Corbyn is that his election four years ago has changed British politics forever, and therefore we need to consider what this all means. We need to consider the election result. If it was a defeat – a defeat that I’m not expecting – then we’d have to look at the scale of that, and where it happened. That requires some calm deliberation.

“So whilst I don’t expect it, I certainly feel that there would have to be a proper debate and discussion.”

Asked who among Labour’s new generation of MPs might contest the deputy leadership, McCluskey named Rebecca Long-Bailey, Angela Rayner, and Laura Pidcock as rising stars.

“I think the deputy leadership will come up, I suppose, at the same time as giving consideration to what has happened in the general election, and there’s lots of young, talented people coming forward,” he said. “There’s lots of young, talented female representatives: Becky Long-Bailey, of course, Angela Rayner, Laura Pidcock, the Lisa Nandys of this world. There’s young males: Dan Carden, Richard Burgon.

“There’s lots of talent, and there’ll be new talent coming through. I know a number of Labour candidates who I believe will be elected, and what an exciting bunch they are: young, committed, radical.”

Separately, McCluskey also predicted that Johnson’s Brexit deal would result in loyalist paramilitary violence, both in Northern Ireland and in Great Britain. “Boris is a little bit like a second-hand car salesman. He didn’t want us to look under the bonnet of his deal to see what the engine was like. And how true that was.”

“The current deal puts a border down the Irish Sea,” McCluskey went on to say. “That’s unbelievably dangerous.”

“Have we got to wait for people to be blown up and killed? Whether these ports are in Stranraer or Pembroke or in Holyhead, or the areas in Northern Ireland where there’s going to be checks. Do the media not understand that loyalist paramilitary groups are meeting almost on a daily basis in Northern Ireland? [It is] incredibly reckless and dangerous.

"Now there’ll be lots of ordinary people who are concerned about other things and don’t see Ireland as a priority for them. But it will be a priority when that violence is brought into the rest of Britain. It’s dangerous, unbelievably dangerous, and we have to call it out."