Theresa May's plan to maintain a soft Irish border after Brexit could be changed to accommodate European Union concerns, her deputy said amid signals that Brussels could reject it.
Cabinet Office minister David Lidington, the Prime Minister's de facto number two, said her high-profile Friday speech was an "ambitious opening bid" for negotiations on trade.
And after Ireland's foreign minister Simon Coveney suggested the EU could block her plans to maintain a soft Irish border while leaving the customs union, Mr Lidington accepted it could be changed to accommodate concerns.
But he rejected suggestions that the UK would have to accept the fallback option of keeping Northern Ireland in an effective customs union with the EU.
The issue is holding up agreement on Britain's exit terms and a transition deal.
Mr Lidington backed Mrs May's plan to avoid a hard border through technological solutions and placing no new restrictions on the 80% of cross-frontier trade carried out by smaller businesses, while suggesting it could be tweaked.
Asked if the backstop option was inevitable, Mr Lidington told ITV's Peston on Sunday: "I'm much less pessimistic than you are, clearly we are at the start of a negotiating period and will want to sit down with our EU partners and work through where their concerns, whether legal or technical, are and see how we might together address this."
But Irish Tanaiste Simon Coveney told BBC One's The Andrew Marr Show he was "not sure that the European Union will be able to support" the plan, as it would be worried about protecting the integrity of the single market.
"While of course we will explore and look at all of the proposed British solutions, they are essentially a starting point in negotiations as opposed to an end point," he said.
Mr Coveney said if agreement cannot be reached during tripartite talks between the UK, Ireland and the EU Commission, the backstop plan of full British alignment with customs union and single market rules that Mrs May "committed clearly" to in December's conclusion of withdrawal negotiations would have to be put in place.
He said: "This isn't a question of either side wanting to put up borders, but if you have to protect a functioning single market, just the same way Britain wants to protect its own single market, well then you have to understand that if goods move from one customs union to another then there needs to be some checks unless there is some mechanism that is negotiated and put in place that prevents that."
Mrs May said she was pleased that Irish PM Leo Varadkar had agreed to form the three-way talks to look at her proposals.
And she declined to defend Boris Johnson's comparison of the border to crossing between London congestion zones in Camden and Islington, but insisted both of them are "absolutely clear" that there will not be a hard border.
Mrs May told Marr: "I think the Irish border is something to which we are all committed.
"We're committed, the Irish government, all the parties in Northern Ireland, to making sure there's no hard border for the future, and that's why I'm pleased that with the Commission and the Irish government we'll be able to sit down and in a very much more detailed sense say, 'What are, you know, the proposals we've put forward, how would they work? Let's see which is the best option for the future."
Meanwhile, Labour former cabinet minister Lord Mandelson dismissed Mrs May's Brexit plan.
He told Marr: "What Theresa May is doing is trying to dance on the head of a pin that simply doesn't exist."
He added: "It will be painful for the country as a result."
Mrs May sidestepped a question over whether a Commons vote on the customs union would amount to a motion of confidence.
An amendment to the Trade Bill calling on the PM to try to negotiate a customs union with the EU, tabled by Conservative former minister Anna Soubry, has attracted the support of enough Tories to threaten defeat for the Government.
But senior Tory Sarah Wollaston, one of the MPs who has signed Ms Soubry's amendment, said the rebels would now give the Prime Minister "breathing space" to try to negotiate her own customs plan.
She told ITV's Peston On Sunday: "I think what we need to see is some kind of partnership that allows frictionless trade at our border and also deals with the situation in Northern Ireland and I want to give her the space to do that."