British Prime Minister Theresa May has warned opponents of her deal to leave the European Union they risk "letting the British people down", a day before a crucial vote in parliament.
She urged critics to give the Brexit deal "a second look", insisting new assurances on the Irish border had legal force.
She said the history books would judge if MPs delivered on Brexit while safeguarding the economy and security.
But Labour leader Jeremy Corbyn said the PM had "completely and utterly failed".
MPs will vote on the terms of the UK's withdrawal from the EU and declaration on future relations on Tuesday evening.
Labour and the other opposition parties will vote against the deal while about 100 Conservative MPs, and the Democratic Unionist Party's 10 MPs, could also join them.
Assistant whip Gareth Johnson has become the latest member of the government to quit his job over the deal, saying in his resignation letter to the PM that it would be "detrimental to our nation's interests".
Ahead of the vote, Mrs May has been briefing MPs on the controversial issue of the "backstop" - the fallback plan to avoid any return to physical Northern Ireland border checks.
She said that while the UK would "never countenance" introducing a hard border, the EU had made it clear that in the event of the UK leaving without a deal, a raft of checks would be introduced "in full".
She said she had won new written assurances from the EU that the contingency customs arrangement being proposed would be temporary and, if it was ever triggered, would last for "the shortest possible period".
Her "absolute conviction" was that the two sides would be able to finalise their future relationship by the end of 2020, meaning the backstop would never be needed.
"I say to members on all sides of this house, whatever you may have previously concluded, over these next 24 hours, give this deal a second look," she said.
"It is not perfect but when the history books are written, people will look at the decision of this House tomorrow and ask 'did we deliver on the country's vote to leave the EU, did we safeguard our economy, security or union or did we let the British people down'."
Mrs May's last-ditch efforts comes amid reports MPs plan to take control of Brexit if her deal is defeated.
Former Labour leader Ed Miliband said the PM must make "herself the servant of the House" if the deal was rejected, giving Parliament an "open and honest process" to express their will.
What happens next?
Tuesday - Day five of debate followed by "meaningful vote" on the PM's deal. MPs will also get to vote on amendments that could reshape the deal. If the deal is rejected Theresa May will get three working days to come up with a "plan B".
Wednesday - Mrs May could head to Brussels to try to get further concessions from the EU.
Monday 21 January - Expected Commons vote on "Plan B".
What has the UK been offered on Northern Ireland?
The so-called Irish backstop will see the UK and EU share a single customs territory until they settle their future relationship or come up with another solution to stop a hard border.
Many Tory MPs, as well as the Democratic Unionists, are adamantly opposed to it.
The EU has given fresh written assurances about how the backstop might be triggered and how long it would last. The key points, in a letter from top officials Donald Tusk and Jean-Claude Juncker to the PM, are:
- The backstop will not affect or supersede the provisions of the 1998 Good Friday Agreement
- The backstop will not extend regulatory alignment with EU law in Northern Ireland beyond what is strictly necessary to avoid a hard border
- Alternatives to the backstop such as "facilitative arrangements or technologies", will be looked at with progress considered every six months after the UK's departure
- Any alternative arrangements would not be "required to replicate" the backstop "provided the underlying objectives continue to be met"
"Were the backstop to enter into force in whole or in part, it is intended to apply only temporarily, unless and until it is superseded by a subsequent agreement," they said.
"The Commission is committed to providing the necessary political impetus and resources to help achieving the objective of making this period as short as possible," it said.
BBC political correspondent Alex Forsyth said the attempt to reassure had not done enough to convince many senior Brexiteers to swing behind the prime minister's deal.
The contentious Northern Ireland backstop remains the biggest sticking point, and nothing short of a legally watertight guarantee that it can't go on indefinitely will be enough for many of those with concerns, he said.
Reports of MPs planning to take over Brexit
The UK will leave the EU on 29 March unless there is a new act of Parliament preventing that.
Because the government controls the timetable for Commons business, it was assumed that this would not be possible.
But three senior Conservative backbenchers are to publish a bill on Monday night that would allow MPs to frame a "compromise" Brexit deal if Theresa May fails to come up with a plan B, Tory Nick Boles has revealed.
Downing Street has said it is "extremely concerned" about the plot, which it says could potentially overturn centuries of Parliamentary precedent.