Former cabinet ministers Iain Duncan Smith and Owen Paterson made a trip to Brussels on Monday to meet European Commission official Michel Barnier.
It came as the prime minister prepared to tell MPs 95% of the UK’s divorce agreement with the EU is now settled, ahead of what could prove to be a hostile reception in the House of Commons on Monday.
Mr Duncan Smith and Mr Paterson met Mr Barnier together with ex-Northern Ireland first minister David Trimble, who backs their plan.
They used the meeting to present the proposals for avoiding a hard border on the island of Ireland after Brexit, but without tying the UK closely to the EU’s single market or customs union.
The plan was dismissed as “dreamland stuff” and “simplistic and ignorant” by Dublin when it was first published by the European Research Group (ERG) of Conservative eurosceptics, which Mr Paterson and Mr Duncan Smith are members of, in September.
Speaking after the meeting, Mr Paterson claimed “using existing techniques, existing processes, and all within existing EU law, we can continue to trade pretty well seamlessly across all borders without damaging the integrity of the EU customs union and single market, which is obviously fundamental to them”.
He described the meeting with Mr Barnier as “long and constructive” but stressed the ERG are “negotiating on behalf of our government”.
Mr Duncan Smith said: “The government negotiates, we’re just here to present a paper – which we did – and have a constructive discussion – which we had – and we’re now going back and we’ll talk to the government about it.”
Denying the trip had undermined Mrs May, the former Tory leader added: “I think this is all within a par of what the government says they want to achieve which is, ultimately, leave on the best terms and the best arrangements.”
Mr Duncan Smith also added his voice to those criticising anonymous Tory MPs who have used violent imagery in discussions about the prime minister’s future.
Mrs May has recently been advised by unnamed Conservatives she is entering “the killing zone” and to “bring her own noose” to a crunch meeting with MPs on Wednesday.
“Whoever did that needs to have the fullest weight of the Conservative Party on them,” Mr Duncan Smith said.
“It has nothing to do with our politics, it has nothing to do with this issue.”
Mr Barnier has made meeting various groups of UK MPs a feature of the Brexit negotiating process, having previously met senior Labour figures as well as leading pro-Remain campaigners.
Elsewhere on Monday, Northern Ireland Secretary Karen Bradley heard fresh warnings of a return to violence if a hard border was to return to Ireland after Brexit.
Irish senator Frank Feighan told a meeting of British and Irish parliamentarians: “We would be very concerned at any border, any installation, that the people of Ireland would absolutely tear down that border with their own hands.
“That’s what’s coming from people, not from me, but from people on the ground.”
Mrs Bradley used the British-Irish Parliamentary Assembly in London to reassert the UK government’s commitment to the Good Friday Agreement and to avoiding a hard border between Northern Ireland and the Republic of Ireland.
At the weekend, an estimated 700,000 marched in London to demand a referendum on the final Brexit deal.
However, Mrs Bradley stated her opposition to a fresh national poll, adding: “I personally think, from my knocking on doors, there would be a bigger vote for Leave.
“The fact is the people spoke. We had a people’s vote.”
In the House of Commons later on Monday, Mrs May will tell MPs the UK’s Brexit divorce agreement with Brussels is now 95% settled.
But the prime minister could face hostility from MPs after she used a Brussels summit last week to pave the way for a possible extension to the Brexit transition period.
The government will also face urgent questions from both sides of the Conservative Brexit divide on Monday.
Leading Brexiteer John Redwood will ask about the costs of staying in the EU’s customs union and the withdrawal agreement.
Tory Remainer Dominic Grieve will then ask about whether the “meaningful vote” on any Brexit deal will be on an amendable motion – giving MPs the option to influence the government’s approach.
Brexiteers may also deliver a show of force to Mrs May later on Monday by backing amendments to the Offensive Weapons Bill.
Or they could save a demonstration of their influence for Wednesday, when Brexiteer amendments to emergency legislation may prevent Northern Ireland being placed in a different regulatory and customs territory from the rest of Britain without a vote in the Stormont assembly.
This is aimed at derailing the EU’s proposal for the Irish border backstop solution.