Len McCluskey has said Labour MPs should vote against Theresa May’s Brexit deal in order to bring down the government.
However he said he feared pro-EU Labour backbenchers could “fail” the left and keep the prime minister in office by supporting her exit agreement.
“My personal hope and belief is that in late Autumn of this year the deal that comes back to parliament will be rejected,” he told the Resolution Foundation think-tank on Tuesday morning,
“It will lead to Theresa May having to resign and it will lead to an early general election in 2019 - that becomes then a referendum.”
McCluskey, a close ally of Jeremy Corbyn, said he would not “rule out” supporting a referendum on the deal if parliament failed to reject it.
But he warned there was a “real danger” of Labour losing support in ‘Leave’ voting areas if it backed a second vote.
Asked in what circumstances he would support a referendum, he said: “The one thing I believe all progressive left people in our nation should be concentrating on, one thing only, removing this government. This 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 a general election.
“If parliament fail in that then I am prepared to look at any other option because I don’t want to wait until 2022.”
He added: “If parliament fail us, and this is my fear, Theresa May might come back with a deal that whilst the Labour leadership might disagree with it, there may be Labour politicians, pro-Europeans who say ’that’s not a bad deal and therefore we are not voting against the government’.
McCluskey said he believed May “will move and more and more to a soft Brexit” which would persuade Labour MPs to support the agreement with Brussels.
“I am relying and hoping that the parliamentary set up we have will defeat it. If they don’t I am prepared to look at any options,” he added.
Parliament has secured the right to have a vote on the eventual Brexit deal the government agrees with the EU.
The concession means MPs will in reality have a choice to either accept the deal or vote to to leave with no deal.
McCluskey was speaking as the prime minister’s government’s flagship Brexit began its passage through the House of Lords.
More than 190 peers are listed to speak in a marathon two-day second reading debate on the EU (Withdrawal) Bill .
While the legislation cleared the Commons last month relatively unscathed – with only one Government defeat – it is likely to face a far rougher ride in the upper chamber which is overwhelmingly opposed to Brexit.
The main battles will come in the weeks ahead when it reaches the committee stage when peers are likely to try to push through a series of major amendments.