Shadow chancellor John McDonnell this morning refused to say whether Labour MPs who back Mr Johnson’s deal would have the party whip withdrawn, effectively expelling them.
He said such issues were a matter for the party’s chief whip, Nick Brown.
But Mr McDonnell added to Sky News: “A three-line whip will apply and we will expect Labour MPs to vote against this deal because it’s such a poor deal.
“I don’t think Labour MPs will want to vote for something that will… sell us out basically.
“It will undermine workers’ rights and make our constituents poorer.”
Mr McDonnell will tomorrow address a rally in Westminster in favour of a second EU referendum.
A staff member for one of Labour’s Leave-supporting MPs claimed Mr Corbyn has given assurances that those who vote for the Brexit deal will not lose the party’s whip.
They said: “This stuff about taking the whip is nonsense, a number of MPs met with Jeremy Corbyn earlier this week and he was clear that they wouldn’t.”