Mr McCluskey’s call came as senior members of Sir Keir Starmer’s shadow cabinet refused to rule out abstaining in a Commons vote expected within days of a deal being reached.
It increases pressure on the Labour leader to order his troops through the Aye lobby in order to ensure a deal is preserved from a potential rebellion by eurosceptic hardliners on the Tory benches.
The Unite general secretary, whose union is the largest financial donor to Labour, said the party should learn the lesson from its confused position on Brexit in the general election of December 2019.
“Frankly, Labour needs on the most important issue of the day, not to be seen to be sitting on the fence,” Mr McCluskey told Times Radio.
“The idea of an abstention, to me, would be completely wrong. I hope they learn the lesson from back in December that any confusion over this will be damaging to them.”
Asked if he felt Labour should vote in favour of any EU trade deal produced by the prime minister, Mr McCluskey said: “Yes, in my opinion, let’s get Brexit done and out of the way.
“Of course it won’t stop us being critical if the deal gives us all kinds of problems going forward. My union has been consistent in wanting to defend jobs, defend investments and try to make certain that we still have an economy that is that is fit for purpose and that can compete against our European competitors.”
Mr McCluskey said that Sir Keir needs to “win the trust of the Red wall seats” and those who voted Leave at the 2016 referendum, and be seen “not to be standing in the way” of what people supported at the 2019 election.
Shadow cabinet ministers including Anneliese Dodds, Emily Thornberry and David Lammy are understood to be pushing for abstention in any vote on a trade deal, in order to ensure that the Conservatives can be fully held to account for the shortcomings of the post-Brexit settlement. But Lucy Powell warned that failing to support a deal would amount to “putting two fingers up” to former Labour supporters in the north who back Brexit.
Sir Keir has acknowledged there are “different opinions” within the party on how to approach a vote. He last week hinted that he is open to backing the PM, saying: “If the choice is a deal or no-deal, then a deal is obviously in the national interest.”
Former Labour prime minister Gordon Brown said it would be up to Starmer to decide what the party does in relation to any deal, telling Sky News’s Sophy Ridge On Sunday: “I don’t think he will make his decision until he sees what the deal is.”
And pro-European backbencher Neil Coyle tweeted: “A trade union leader parroting Tory Johnson’s mantra word for word.
“Given how wrong Len has been on so much else, this is just another red flag (so to speak) on why voting for Johnson would be a mistake.”
Shadow home secretary Nick Thomas-Symonds declined to say which way the part would vote.
But asked whether abstaining remains on the table, he told Ridge: “Of course there are various options.
“When you get any particular vote that you get before parliament there are the options that there always are – that’s absolutely the case.”
Asked on BBC1’s Andrew Marr Show if Labour would back any agreement on the future relationship, shadow Cabinet Office minister Rachel Reeves replied: “Let’s see.
“At the moment the priority is to get a deal but we’ll have to look at the content of a deal but also any legislation that comes to Parliament.
“We’re not going to give them a blank cheque but I think I have been very clear … that the most important thing is the government gets a deal.”