Jeremy Corbyn’s key union supporter, Unite’s Len McCluskey, sparked a backlash on Tuesday as he told the Labour leader that victory in the general election means taking a tough line on free movement of workers.

In a Guardian interview, the Unite general secretary said shadow cabinet members should not upset Labour’s carefully crafted Brexit position during the election and that he would oppose any attempts to extend free movement as voted for at the party’s annual conference in Brighton.

“We will have to see what’s in the manifesto, but I don’t think [what conference voted for] is a sensible approach and I will be expressing that view,” McCluskey said, adding that he was keen to shore up the party’s support in marginal seats in the Midlands and north of England being targeted by Boris Johnson.

With the Conservatives seeking to make migration a key election issue, McCluskey said Labour needed to show how it was going to prevent pay and conditions from being undercut before it could consider relaxing its stance. “It’s wrong in my view to have any greater free movement of labour unless you get stricter labour market regulation.”

His intervention, ahead of Saturday’s meeting to sign off the manifesto, infuriated activists who campaigned for the radical pro-migration motion passed at conference.

Alena Ivanova, from the Labour Campaign for Free Movement, said: “A Romanian care worker and a British bus driver have more in common with each other than they do with their boss. That is the basis of the trade union movement. Len MCluskey’s job is to fight for their full rights, for decent pay and the right not to be deported and harassed by the state because of their immigration status.”

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Several senior Labour figures privately expressed concern about McCluskey’s approach and said they expected his view to prevail at Labour’s manifesto meeting this weekend.

“Len has re-emerged as the flagbearer of the old Labour right. He’s part of a backward-looking, small-c conservative nostalgia-tripping wing of the labour movement that wants ‘trade union rights for British workers’,” warned one Labour candidate hoping to retain their remain-voting seat.

Unite is Labour’s biggest affiliate and will have considerable influence as the election manifesto is finalised over the coming days.

In a wide-ranging interview, McCluskey said:

Labour needed to get the election debate off Brexit and on to the day-to-day issues that really mattered to voters.

The shadow cabinet should keep quiet about how it would campaign in the event of another referendum.

White working-class supporters of leave in the 2016 referendum would be driven into the arms of a hard-right party unless their concerns about migration were dealt with.

There would be no attempt to move Labour back to the centre if the party lost the election.

He predicted paramilitary attacks on UK mainland ports as a result of the customs border down the Irish Sea that forms a key part of Johnson’s Brexit withdrawal deal.

McCluskey said his union had campaigned on a platform of remain and reform in the 2016 referendum. “The reform part was important. The idea that we were happy with Europe is a joke. We clearly weren’t.”

He said Labour’s Brexit strategy – to renegotiate a deal and then hold a confirmatory referendum with an option on the ballot paper to remain – would cut through if it was expressed often enough and clearly enough. “Labour’s task is to get beyond Brexit and get on to issues that affect ordinary people on a day-to-day basis. I believe Labour can do that.”

He said members of Corbyn’s shadow cabinet should not prejudge how they might campaign in a second referendum and must remain disciplined. “I am calling on some members of the shadow cabinet who are passionate remainers that they shouldn’t declare now which way they will campaign.”

McCluskey said: “We have to ask: why did so many people vote to leave? Too many of those who live in metropolitan political and media circles don’t really grasp why people voted the way they did.”

He said one reason for the leave vote was deindustrialisation, with people living in the “forgotten towns and cities” not just asking “what has Europe done for us” but also keen to give the political elite “a slap in the face”.

“The other reason was migrant labour coming to the UK from Europe. If you don’t understand those concerns, you fail to grasp the divisions that exist. Migrant workers are to blame for absolutely nothing in this country. They are just trying to better their lives and the lives of their families. It’s the greedy bosses that are using them to undercut pay and conditions.

“If we don’t deal with the issues and concerns, we will create a vacuum that will be filled by a far right seeking to become the voice of the white working class.”

McCluskey said he did not agree with the view expressed by some on the left that complete free movement of capital should be matched by complete free movement of labour. “The only beneficiaries are the bosses of unscrupulous companies.”

Corbyn was challenged about McCluskey’s comments on a campaign visit to Scotland, but declined to be drawn. “We’re going to have our clause V meeting at the weekend and no doubt that issue is going to be discussed there,” he said.

“There are massive job shortages in the NHS across the whole of the UK – I think there are something like 30,000 to 40,000 nurse vacancies – there’s also a shortage of doctors across the whole of the NHS, and EU workers have made a massive contribution to our society.”

Labour could form the next government, McCluskey said, provided it could persuade leave and working-class voters that Labour was on their side. If people thought the Tories were suddenly going to remember the forgotten towns and stop the influx of cheap labour then they were living in “cloud cuckoo land”, he said.

McCluskey said he expected Corbyn to be Britain’s next prime minister but predicted there would be no drift back to the centre even if Johnson remained in Downing Street because nobody in the party was offering an alternative ideology to the leader’s anti-austerity stance.

“I don’t think there will be a retreat from the left. Jeremy Corbyn changed British politics forever four years ago,” he said.

“I am not expecting Jeremy to lose, but whoever comes after him and whenever that happens, it will be somebody committed to making Britain a fairer, juster country. The shift is for good.”

McCluskey said ports on the UK mainland such as Holyhead and Stranraer would be targets for loyalist paramilitaries as a result of a customs border in the Irish Sea that forms part of the withdrawal agreement negotiated with the EU. “Do we have to wait for people to be blown up for someone to recognise this?”