Labour would vote for a second referendum to be added to any deal proposed by Boris Johnson on Saturday, a shadow minister has said.

Jenny Chapman, the shadow Brexit minister, said Labour would push for a confirmatory vote along with other opposition parties and Tory rebels.

The move would mean Labour prioritising a second referendum over a general election, despite Jeremy Corbyn having repeatedly said he would favour the latter.

Chapman told the BBC’s Andrew Neil: “The expectation would be: should a deal be tabled on Saturday, I’m as sure as you can be that there will be an amendment tabled that would want to see a referendum attached to the deal. I would expect us to support that.”

Pressed on why Labour would support a second referendum, keeping Johnson in power for months to come, rather than an election, Chapman said: “I would rather have a general election but we are not in control on this. So should that opportunity come on Saturday, to have that referendum on a deal … the pragmatic, sensible thing for the Labour party to do, given we’ve been asking this, would be to take that opportunity.”

Her remarks indicate a shift in position since Corbyn suggested he would only support a referendum on a deal that accorded with Labour’s “five pillars” including a customs union and more protections for workers’ rights. The Labour leader also cautioned MPs as recently as this weekend against the idea of a referendum on a Johnson-backed deal.

Asked whether Corbyn had been told of the position, Chapman said: “He’s told me this … I’m telling you today what the situation is. I think on Saturday if there is that opportunity that the Labour party will take it.” Asked if Labour would go for a general election if it cannot get a referendum, she said: “We’ll see.”

Labour MPs pushing for a second referendum said they believed the numbers would now be pretty close as many pro-deal backbenchers on both the Labour and former Tory/independent side are balking at the hardness of the Brexit being proposed by Johnson.

Labour sources said the party’s position would depend on the amendment but there could be a motion that allowed it to support a referendum while making clear MPs rejected Johnson’s Brexit deal.