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.”