Opposition party leaders have penned a joint letter to Theresa May calling for guarantees of a "meaningful vote" on the terms of Brexit.
Jeremy Corbyn, Sir Vince Cable, the SNP's Ian Blackford and Plaid Cymru's Liz Saville Roberts all signed the letter which was posted on Tuesday.
The signatories warned the Government against attempts to "muzzle Parliament" when it comes to examining the Brexit deal.
It comes after Downing Street confirmed that a deal on a "technical level" had been struck with the EU.
The government has previously promised that Parliament would get some form of vote on the final terms of any deal.
The letter read: "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.
"We want to be clear that this would be wholly unacceptable."
The signatories said a "much more extensive" debate on the arrangements for the vote was required than the 90 minutes usually set aside for a business motion.
And when the actual motion is considered, they said there must be opportunities to table "multiple amendments".
"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," they said . "We believe Parliament must be allowed to express its view and hold the executive to account.
"This would not be possible if Parliament was unable to table, debate and consider amendments before any decision on the substantive motion."