Video report by ITV News Political Correspondent Libby Wiener
Theresa May has warned the UK will be "back at square one" if her Brexit deal is defeated in Parliament.
Taking questions from the public on a live radio phone-in, the Prime Minister called on MPs - and the rest of the country - to come together and support the deal she is hoping to get agreed at a Brussels summit this weekend.
Mrs May rejected the idea that the Government could go back to the EU and renegotiate a better deal.
"If this deal doesn't go through we are back at square one. What we end up with is more uncertainty and more division," she told one caller on BBC Radio 5 Live.
"I believe that if we were to go back to the European Union and say 'People didn't like that deal can we have another one?' I don't think they are going to come to us and say 'We will give you a better deal.' This is the deal that I think works for the UK," she said.
Mrs May wouldn't give a direct answer when repeatedly pushed on what the plan would be if the deal was rejected.
"Personally there is no question of no Brexit because the Government needs to deliver on what people voted for in the referendum in 2016," she said.
Meanwhile, Spain has threatened to "veto" the Brexit deal if its concerns about Gibraltar are not dealt with.
European Union leaders will gather on Sunday for a special summit where they are expected to back the agreement.
But Spanish premier Pedro Sanchez demanded last-minute changes to the deal after speaking to Theresa May on Wednesday, tweeting: "After my conversation with Theresa May, our positions remain far away. My Government will always defend the interests of Spain. If there are no changes, we will veto Brexit."
On Friday a spokesperson for the Prime Minister said: "We've negotiated openly and constructively with the EU on Gibraltar and worked closely with Spain - we are making sure we agree a deal that works for the whole of the UK family.
"The withdrawal agreement isn't being reopened. We will work with the governments of Gibraltar and Spain."
Mrs May faces an uphill task to get MPs to back her deal, which former Brexit secretary Dominic Raab said was "worse" than staying in the EU.
On Thursday, ITV News Political Editor Robert Peston told News at Ten that a "growing number" of MPs now back another referendum and "there is a reasonable prospect that we will end up staying in the EU".
Spain cannot “veto Brexit” or the Withdrawal Agreement, but a refusal to co-operate will sour the atmosphere at a summit where leaders of the 27 remaining EU members are aiming for a unanimous decision.
Spain’s complaint is about an article in the Withdrawal Agreement, the 585-page legal document.
ITV News Correspondent Angus Walker said: "The Spanish accuse Britain of sneaking in Article 184, which says the UK and EU will work with best endeavour to get a future trade deal.
"Spain thinks that ignores the need for any deal on Gibraltar being subject to bi-lateral agreement between Spain and the UK."
Mrs May will head to Brussels on Saturday for eve-of-summit talks with Jean-Claude Juncker knowing that she faces an uphill task in Westminster to persuade her own MPs to back her deal.
Mr Raab, who resigned last week in protest at the terms of the agreement, said he would not advocate staying in the EU, but added: "If you just presented me terms, this deal or EU membership, because we would effectively be bound by the same rules but without the control or voice over them, yes, I think this would be even worse than that."
He told BBC Radio 4's Today programme that ministers should contemplate leaving without a deal, saying: "We will, I think, inevitably see Parliament vote this deal down. And then I think some of those other alternatives will need to come into play."
Labour MP David Lammy was among the Remain-supporters to seize upon the former minister's comments.
After declaring that a final agreement on Brexit is “within our grasp” following a breakthrough on future relations between the UK and EU, Mrs May endured a bruising session in the House of Commons as critics lined up to condemn both the divorce deal contained in the Withdrawal Agreement and the aspirations for a close future relationship in the Political Declaration.
Former foreign secretary Boris Johnson urged her to “junk” her backstop plan for keeping the Irish border open, which he said “makes a nonsense of Brexit”, while former Cabinet minister Priti Patel branded the deal “a costly surrender by the UK Government”.
With so many of her own MPs against the deal, the DUP highly unlikely to support it and Labour also against it, Mrs May looks almost certain to see her agreement voted down in the Commons.
ITV News Political Editor Robert Peston said a "growing number" of MPs are now suggesting the "only option" is to stay in the EU through another referendum.
Downing Street told Peston on Thursday that Mrs May would not stay on as prime minister if Parliament decided there should be another referendum.
Peston told ITV News at Ten: "For the first time I think there is a reasonable prospect that we will end up staying in the EU.
"It's not just what MPs are saying to me, but also when I talk to other European capitals that is what they now think."
Mrs May said the joint UK/EU declaration on the future relationship is “the right plan for the UK”.
The text calls for an “ambitious, broad, deep and flexible partnership” between the EU and the UK in areas like trade, security and defence, possibly in the form of a Ukraine-style Association Agreement.
It confirms the UK’s right to develop an independent trade policy and end the free movement of EU nationals and leaves open the possibility of using technological solutions to keep the Irish border open after Brexit.
But elements which infuriated Brexiteers included:
Plans for a “free trade area, combining deep regulatory and customs co-operation”, building and improving on the “single customs territory” provided for in the withdrawal agreement
Provisions to ensure a “level playing field” on business competition, which could cover areas including state aid, workplace and environmental protections, climate change laws and tax
A role for the European Court of Justice in providing “binding” rulings on the interpretation of EU law in any disputes between the two sides
Downing Street has always stressed that the 585-page legally binding Withdrawal Agreement setting out the terms of the UK’s departure from the EU – including a “divorce bill” estimated at £39 billion – can only be finalised alongside the shorter declaration setting out the two sides’ aspirations for their future relations.