The Unite leader Len McCluskey has said Labour must 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 trade union boss, a key ally of Jeremy Corbyn whose views will carry significant weight in writing any future party manifesto, said Labour had to address how it would approach negotiating future trade deals if the UK had left the EU after Labour came to power.

“I believe that if we haven’t left by 31 October that we should respect the 2016 referendum, which means leaving Europe but on a deal that the 48% would be happy with,” McCluskey told the BBC. “That is access to a frictionless market and a customs union protecting jobs and investment.

“And the 52% would be happy because of the policies that Labour have to invest in their communities and to deal with the abuses of migrant labour coming into our country with labour market regulations.”

He said a no-deal Brexit would be “almost impossible to stop” and said he was not certain a no-confidence vote in Boris Johnson would be successful.

“The important thing for me is for Labour to continue to hold a very clear vision of the type of Britain we want to be and the relationship we want with Europe, because come the 31 October if we leave without a deal, there is still going to have to be debate and discussion about trade deals and about what kind of relationship we have,” he said.

McCluskey said that a referendum would be “a legitimate stance” in order to test the popularity of no deal with the British public but said he did not believe it would take place.

“It’s not going to happen, of course – there isn’t going to be a second referendum under Boris Johnson’s no-deal approach to life,” he said.

Asked if that went against so many Labour supporters and MPs who wanted another referendum, McCluskey said: “So many Labour MPs and supporters want the issue done and dusted.”

McCluskey said the trade unions had endorsed a position that Labour should negotiate a Brexit deal if they came to power prior to the UK leaving.

The joint position, agreed by union leaders in the aftermath of disappointing results for Labour in the EU elections, also suggested any Labour-brokered deal be put to a public vote, but the party would not commit to campaigning for remain against its own Brexit deal.