Theresa May has said a rejection of the deal would pose "grave uncertainty for the nation"
A group of British MPs has delivered a sharply critical verdict on Theresa May's Brexit deal.
In a new report, the cross-party Committee on Exiting the European Union claims that the Withdrawal Agreement and Political Declaration do not offer enough clarity or certainty about the future.
The report raises concerns about the Irish border situation, including efforts to ensure there is no hard border.
The MPs state: "In December 2017, we said that we did not see how it would be possible to reconcile maintaining an open border on the island of Ireland with leaving the Single Market and Customs Union, which would inevitably make the Northern Irish border the UK’s customs and regulatory border with the European Union.
"Since then, we have seen no realistic, long-term proposals from the Government that would address this."
They also suggest that 'uncertainty remains' for EU citizens in the UK, and insist that it would be 'unacceptable' if a planned White Paper on future immigration policy is not published before Tuesday's 'meaningful vote' on the deal in the House of Commons.
The committee report adds: "It is clear that after 20 months of intense negotiations, only the terms of the UK’s exit are fully known; the nature of the future relationship with the European Union is not, and therefore does not provide long-term certainty."
Theresa May could face a heavy defeat on Tuesday amid deep divisions over the Brexit withdrawal agreement - including within her own Conservative party.
The Brexit Secretary says the vote will take place on Tuesday as planned, despite the prospect of a major defeat.
Stephen Barclay - who only recently took over the role, following the resignation of his predecessor Dominic Raab over the deal - said Mrs May's government is fully focused on the vote in two days time.
He acknowledged that the deal isn't perfect, but it's the best to have come from 18 months of negotiations.
He told BBC's Andrew Marr Show: "The vote is going ahead, and that's because it is a good deal - it's the only deal.
"It's important that we don't let the perfect be the enemy of the good."
Speaking to the Mail on Sunday, Mrs May herself claimed the UK 'would truly be in uncharted waters' if MPs reject the deal.
She argued: "It would mean grave uncertainty for the nation with a very real risk of no Brexit.
"We have a leader of the opposition who thinks of nothing but attempting to bring about a general election, no matter what the cost to the country… I believe Jeremy Corbyn getting his hands on power is a risk we cannot afford to take."
Speaking earlier this week, Mr Corbyn insisted his Labour party "is ready to step in and negotiate a Brexit deal that works for the whole of our country".