The risk of accidentally crashing out of the EU without a deal has been described as “very high” by a key EU architect of the Brexit deal, with parliamentary backing for changes to the backstop likely to be met with a brick wall in Brussels.

Senior Conservative MPs are seeking to form a majority in a Commons vote on Tuesday calling for Theresa May to demand an alternative plan to the Irish backstop for avoiding a hard border on the island of Ireland.

But on Monday, EU officials and diplomats said the amendment tabled by the Tory MP Graham Brady, and backed by Downing Street, failed to offer any clue as to what alternative arrangement parliament could support.

With the votes on Tuesday unlikely to offer any clarity on what MPs can unite behind, the EU’s deputy chief negotiator, Sabine Weyand, offered a sober analysis of the chances of a deal being ratified in Westminster.

She said: “We need to have a majority that doesn’t just get agreement over hurdle of a meaningful vote by a narrow majority but we need to have a stable majority to ensure the ratification. That’s quite a big challenge. There’s no negotiation between the UK and EU – that’s finished.

“There’s no point beating about the bush – the agreement was defeated with a two-thirds majority in the House of Commons. That’s a crushing defeat by any standards. It’s quite a challenge to see how you can construct out of the diversity of opposition a positive majority for a deal.”

Weyand said of the two years of talks due to end on 29 March: “There’s a very high risk of a crash out not by design, but by accident. Perhaps by the design of article 50, but not by policymakers.”

“We think we can handle it,” Weyand said. “I’m less sure about UK side. For us it’s about EU-UK trade relationship and disruption to supply chains. For the UK a no deal would mean that a part of the regulatory and supervisory structure of economy breaks away – a much bigger challenge.”

In an apparent sign of the frustration in Brussels at May’s handling of the negotiations, the German EU official also contrasted the transparency in Brussels on their goals to the approach taken by Downing Street.

Speaking at a thinktank event in Brussels, she said: “You cannot lead a negotiation like that in secrecy. We’ve seen on UK side the fact this was handled in a very small circle and that there was no information about all the things that were tried in the negotiations is now a big handicap.”

The prime minister is set to return to Brussels in the coming days but Michel Barnier’s deputy offered little succour to those in London hoping for a renegotiation of the withdrawal agreement containing the backstop, an arrangement under which the whole of the UK could stay in a customs union in order to avoid a hard border.

The amendment tabled by Brady, the chair of the backbench 1922 Committee of Tory MPs, commits MPs to backing the withdrawal agreement should the prime minister secure a replacement of some sort for the backstop.

But some Brexiters have said they will only vote for the withdrawal agreement if the Irish backstop is replaced with a simple statement of intent over avoiding a hard border.

Other MPs, including Brady, are in favour of drafting a separate legal document detailing how the UK might in certain circumstances extract itself from the customs union envisaged in the backstop.

“We’re not going to reopen the agreement,” Weyand said at the European Policy Centre event. “The result of the negotiation has been very much shaped by the UK negotiators, much more than they actually get credit for. This is a bit like snatching defeat from the jaws of victory. The backstop was very much shaped by UK.”

“It feels like Groundhog Day,” Weyand added of others in Westminster calling for a time limit or unilateral exit clause in the backstop. “None of this is new. This has been extensively discussed at the negotiating table amongst the EU27. EU27 were unanimous a time limit to the backstop defeats the purpose of the backstop.”

Of the suggestion that there was a technological solution for avoiding a hard border, Weyand said: “We looked at every border on this earth, every border EU has with a third country – there’s simply no way you can do away with checks and controls.

“The negotiators have not been able to explain them to us and that’s not their fault; it’s because they don’t exist.”

She added that in the event of a no-deal Brexit, the UK and Irish governments would “lose the one operational solution to the Irish border conundrum but we’d be stuck with the same scenario”.

“So you could imagine a no-deal scenario with the backstop being discussed,” she said.

Weyand further warned that the debate in Westminster, in which discussions over the rival strengths and weaknesses of Norway and Canada’s relationship to the bloc have recently dominated, appeared at times to be “uninhibited by any knowledge of what is actually in the withdrawal agreement”.

“Our impression is discussion is much more about the future of the country and the future of the UK-EU relationship than about the content of the withdrawal agreement,” she said.

A separate amendment due to be voted on, tabled by the Labour MP Yvette Cooper, would force the prime minister to delay Brexit beyond 29 March should she have failed to ratify a deal by the end of February.

Weyand said the EU’s heads of state and government would need information on “the purpose of an extension”. “The idea of going into serial extensions really isn’t very popular in the EU27,” she added.