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 - allowing them to add conditions and influence the shape of the deal.
The vote, most likely to come later this year, is gearing up to be the biggest showdown in the lengthy negotiations to leave the EU, Britain's biggest shift in foreign and trade policy in almost half a century.
The government last month said any attempt to change the wording of a vote will not impact the agreement itself or prevent or delay Britain's departure.
"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 opposition leaders said in the letter.
"While we recognise parliament will have to approve or disapprove any agreement, it would be reckless to present this vote as take-it-or-leave-it without parliament being able to suggest an alternative." (Reporting by Andrew MacAskill; editing by Guy Faulconbridge and John Stonestreet)
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