Labour Party leader Jeremy Corbyn, Ian Blackford, leader of the Scottish National Party in the British parliament, and Liberal Democrat leader Vince Cable called in a joint letter for the right to put down amendments before the vote - effectively giving parliament a vote on an alternative path.
Last month, the government said parliament must first either accept or reject the divorce deal and it should not automatically be subject to amendments.
"Recent interventions from government ministers have suggested that you and your government may seek to limit or constrain the process on the final vote, in an attempt to muzzle Parliament," the leaders said in the letter. "We want to be clear that this would be wholly unacceptable."
(Reporting by Andrew MacAskill; editing by Guy Faulconbridge)
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