Labour MPs should oppose any Brexit deal that Theresa May puts to parliament this year in order to help trigger a general election and cause her downfall, Len McCluskey, the Unite general secretary, has said.

Len McCluskey, whose union is Labour’s biggest trade union backer, said his hope was for Labour to join other MPs in rejecting the deal, which was unlikely to be in the best interests of the people.

He said he did not support the idea of a second referendum at this point, but did not rule out backing one if MPs failed to reject the prime minister’s settlement, which is to be put to parliament in the autumn.

On the question of a second public vote, McCluskey told an event for the Resolution Foundation thinktank: “I personally wouldn’t rule that out. I’m not saying I’m in favour of it, or my union is in favour of it. At the moment we are in favour of parliament scrutinising the deal.

“My personal hope and belief is that in later autumn the deal that comes back to parliament will be rejected. It will lead to Theresa May having to resign and it will lead to an early general election in 2019. That then becomes a referendum.”

His intervention comes at a time when Labour has said it opposes a second referendum and has not yet publicly said how it would vote on any Brexit deal.

Were Jeremy Corbyn’s party to vote against any deal struck by the prime minister, there would only need to be around a dozen Conservative rebels to bring down the deal and cause chaos for May.

Opposition from Conservatives could either come from pro-Europeans if the deal looks like a hard Brexit, or the Eurosceptic right if they think the agreement keeps the UK too close to the EU.

Opposing the deal would doubtless lead the Conservative leadership to accuse Labour of trying to scupper Brexit, despite the latter having voted to trigger article 50.

Asked by the Guardian in what circumstances he would support a second referendum, McCluskey said: “The one important thing for me, that I believe all progressive left people should be concentrating on, is removing this government. That should concentrate all our minds.

“How quickly can we remove this government? I believe it can be done by parliament rejecting the deal that comes back and forcing an election. If parliament fail in that, I am prepared to look at any other option. Because I do not want to wait until 2022.”

He added: “At the moment, I am clear: late autumn, [the deal] goes to parliament, parliament defeats it and we get an early general election. If parliament fails, and this is my fear, Theresa May might come back with a deal that whilst the Labour leadership disagrees with it, there may be Labour politicians – pro-Europeans – who say: ‘That’s not a bad deal and therefore we’re not voting against the government.’ I believe Theresa May will move more and more to a soft Brexit. So I’m relying and I’m hoping that the parliamentary setup will defeat it. If they don’t, then I’m prepared to look at any options.”

He later tweeted to clarify: “Just to be absolutely clear. I am NOT calling for a 2nd Brexit referendum. I agree with Lisa Nandy [a Labour MP who was also on the panel] that we need to understand the reasons behind the first vote. My preferred approach is for parliament to vote down a Tory deal because it won’t be in the interests of the people.”