Jeremy Corbyn has warned members of his top team it will be "impossible" to keep their jobs if they vote against triggering the start of Britain's divorce from the European Union (EU).
The Labour leader is facing a growing revolt over Brexit after two resignations from his front bench, with more expected to come.
Mr Corbyn said he had made it clear that Article 50 "has to be implemented" but insisted the party planned to secure crucial amendments.
He told ITV One's Peston on Sunday: "There was no need for anyone to resign at this stage.
"It's obviously impossible to carry on being in the shadow cabinet if you vote against a decision made after a very frank and very long discussion of the shadow cabinet earlier this week."
Jo Stevens became the first member of the shadow cabinet to quit after the party leader said he would impose a three-line whip requiring MPs to support the European Union (Notification of Withdrawal) Bill.
Standing down as shadow Welsh secretary, she said Brexit was a ''terrible mistake''.
Her resignation followed Tulip Siddiq's announcement she was quitting as a shadow education minister and two Labour whips - meant to enforce party discipline - indicated they would rebel in next week's vote.
Labour's deputy leader Tom Watson said the divisions in the party would be handled sensitively and suggested some of the rebels could be back in senior roles within months.