Unite’s general secretary, Len McCluskey, has suggested that his union will protect any Labour MP who votes for a new Brexit deal if they face deselection by party members – even MPs on the party’s right.

The remarks from the trade union boss come ahead of a key speech by Jeremy Corbyn in Salford in which the Labour leader will appeal to leave voters, saying efforts to stop no deal were about confronting those “hijacking the referendum result”.

Corbyn will warn of the threat of a no-deal exit from the European Union, ahead of parliamentary efforts by Labour and Tory rebels to stop Boris Johnson crashing the UK out of the bloc.

He will say: “The battle to stop no deal isn’t a struggle between those who want to leave the EU and those who want continued membership. It’s a battle of the many against the few who are hijacking the referendum result to shift even more power and wealth towards those at the top.”

Though Labour has made it explicit that it will campaign for another Brexit referendum, including backing remain against any Conservative-backed deal, McCluskey has been one of the key sceptical voices.

The Unite boss, a close ally of Corbyn, said MPs should not be targeted if they backed Brexit and he would make his views known if members wanted to deselect them on that basis.

McCluskey said he had believed a cross-party deal could have been struck between the Labour leader and Theresa May, but he now thought enough rebel Labour MPs would vote for a new deal to allow it to pass, if it could be brokered by Johnson.

In an interview with Prospect magazine, McCluskey said many Labour MPs would have voted for May’s deal if they believed it stood a chance of passing. “They weren’t going to break the whip for no reason,” he told the magazine.

Labour MPs are facing so-called trigger ballots this autumn, where members have the opportunity to vote for a full reselection process in their constituencies, which would pit incumbent MPs against alternative candidates.

A number of Labour MPs, including Stephen Kinnock, Sarah Champion and Gareth Snell, have suggested they either regret not voting for May’s deal or could vote for any version of a deal now to avoid both no deal or a second referendum.

McCluskey said Unite would have spoken out to defend any MP who had followed though – even those at the opposite end of the party’s ideological spectrum. “I would have made my views known to our members within that constituency,” he said. “I would have been prepared to publicly say: ‘Well, hang on, if this is why you’re attacking this MP, then in my opinion you’re wrong.’”

McCluskey said he did not believe Johnson would secure a new deal, but said more MPs could back it if he did. “If that happened, I think you’re right, but I don’t think it’s going to happen.”

Kinnock, the Labour MP who coordinates aroundabout 30 MPs in a group called Respect the Result, has previously said he believed there was increasing feeling among many of his colleagues opposed to a second referendum that passing the withdrawal agreement bill was the best option. McCluskey has previously argued that Labour must be prepared to support a pragmatic Brexit deal, saying it appeared to be impossible to stop no deal and that there was no path to a second referendum. The majority of Labour MPs who are sceptical about a referendum are still likely to back the cross-party efforts to pass a bill this week that would mandate Johnson to seek and extension to Article 50.

Corbyn will convene a special meeting of the shadow cabinet on Monday to lay out the party’s plans, which have been coordinated with senior Conservatives opposed to no deal.

Speaking at an event in Salford on Monday, the Labour leader will say: “We are working with other parties to do everything necessary to pull our country back from the brink.

“Like all progressive change, democracy was won from below, it wasn’t handed down from above. So, when a prime minister who hasn’t won an election and who doesn’t have a majority decrees that parliament will be shut down because he knows his plan for a disastrous no-deal doesn’t have the votes, we say that is an attack on democracy which will be resisted.”

Corbyn will say that a no-deal scenario is a “Trump deal” one. “A no-deal Brexit is really a Trump-deal Brexit, leading to a one-sided US trade deal that will put us at the mercy of Donald Trump and the big American corporations.

“The pally enthusiasm of the US president for Britain’s new prime minister only underlines that in Boris Johnson, Trump has found a compliant British leader who will dance to his tune.”

Should plans to legislate against no deal fail, the shadow chancellor, John McDonnell, suggested that Labour could attempt a no confidence vote in the government. “Our view is we have to use every mechanism we possibly can to prevent a no deal, and that clearly is still on the table,” he told Sky News. “Let me just say, general election? Bring it on. At the last general election, we were 24 points behind and we virtually drew level with the Conservatives.”

Keir Starmer, the shadow Brexit secretary, sounded a note of caution over plans to trigger an election, saying the party wanted to avoid handing Johnson an opportunity to call an election after 31 October. “We’ve been thinking long and hard about that because we have to have a way of ensuring that we don’t crash out during the course of an election,” he told the BBC’s Andrew Marr show.

“I’m not going to get into the details, but we are absolutely on that point. That is a judgment call for Jeremy Corbyn to make when he thinks the time is right. The focus for this week is on legislation.”

Starmer said Corbyn should have the first opportunity to form a government if a no-confidence vote was successful, but did not rule out supporting another candidate. “If we win a vote of no confidence, it is for the leader of the opposition to say he can form a government … If he can’t, we’ll address that when we get there,” he said.