Theresa May will probably lose a Commons vote on her Brexit deal, former Brexit Secretary David Davis has said.
But Mr Davis - who quit his cabinet role over the Brexit plan in July - said he believed defeat would prompt the UK and EU to agree a "better deal".
He also said the UK had hundreds of plans ready in case the country leaves the EU without any agreed Brexit deal.
Mr Davis said there might be "some hiccups" but the UK was "a big country" and "we can look after ourselves".
Brexit is due to happen on 29 March 2019, as a result of the referendum in June 2016 in which people voted by 51.9% to 48.1% for the UK to leave the European Union.
Although 95% of a Brexit deal is said to be agreed, the UK and the EU have yet to agree on how to guarantee that there will be no return to a visible border between Northern Ireland and the Republic of Ireland in future.
Both sides agreed to put in place a "backstop", also referred to as an insurance policy, that would only be triggered if a future trade deal is not in place by the end of 2020 - or if this final deal does not ensure a "frictionless" border.
This is because there have been warnings that a return of visible border checks could undermine the peace process in Northern Ireland, as well as damaging businesses operating on both sides.
One option for the backstop is for the whole of the UK to remain temporarily aligned to the EU's customs union, avoiding the need for customs checks at the border.
But how long this would last, and how the arrangement could be terminated, has not yet been settled. The government is under pressure from some Tories, as well as Labour, to publish the precise legal advice about how the arrangement would work.
If a Brexit deal is agreed between the UK and the EU, it then has to be approved by the House of Commons and the 27 remaining EU member states.
Mr Davis told BBC Radio 4's Today programme that defeat in the Commons for the current plan was "looking like a probability", especially if MPs were not shown the full legal advice on the Irish border backstop plan.
He said MPs needed to see the "complete legal advice, not a summary" before voting on such a crucial issue.
Mr Davis added: "Are we going to have to wait until the Irish government says it's OK to leave? If so, that's not acceptable.
"Are we going to have to wait until it's convenient for the (European) Commission to say when we leave? If so, it's not acceptable.
"I suspect that they have not pinned down any of these issues and they need to be pinned down before Parliament votes."
In other Brexit developments...
International Trade Secretary Liam Fox said that the UK government must have the right to decide when to leave any backstop which involves a customs arrangement.
It is understood that the Cabinet has discussed using a mutual (UK and EU) review mechanism to leave any backstop.
When asked if a system requiring a mutually agreed withdrawal from a backstop arrangement would be of concern, Mr Fox said that the government "had an instruction from our voters to leave the European Union and that decision can't be subcontracted to somebody else. That needs to be an issue for a sovereign British government to be able to determine".
UK Foreign Secretary Jeremy Hunt delivered a speech in Paris, in French, in which he said the UK and France would remain tied by "bonds of friendship" after Brexit.
On the issue of publishing the full legal advice, he said: "We have an excellent attorney general and he is giving very good legal advice on everything that we are considering signing up to, so we are going into this with our eyes open".
Mr Hunt also said that it was "entirely possible to reach an agreement" within the next three weeks, although to reach agreement within the next week might be "pushing it a bit".
In Dublin, the Irish foreign minister Simon Coveney told the Irish Canada Business Association conference: "I would urge caution that an imminent breakthrough is not necessarily to be taken for granted, not by a long shot.
"Repeatedly people seem to make the same mistake over and over again, assuming that if the British Cabinet agrees something, well, then that's it then, everything is agreed.
"This is a negotiation and needs to be an agreement of course between the British Government but also with the European Union and the 27 countries that are represented by Michel Barnier and his negotiating team.
"So while of course we want progress to be made and we want it to be made as quickly as possible because time is moving on, I would urge caution that people don't get carried away on the back of rumour in the coming days."