Concerns have been raised that the Luxembourg prime minister’s humiliation of Boris Johnson will harden negotiating positions in London.

The decision by Xavier Bettel to proceed with a press conference after the British prime minister had declined to appear in front of anti-Brexit protesters prompted criticism from UK politicians.

The justice secretary, Robert Buckland, accused Bettel on Tuesday of being involved in a “media stunt”. And the former Conservative leader Iain Duncan Smith said the episode would “feed people’s dislike and distrust of the EU”.

“This arrogant and ill-mannered behaviour betrays your motives,” the MP tweeted.

According to a report, the US ambassador to the UK also thought the events in Luxembourg had strengthened British resolve to leave the EU. The Daily Telegraph said Woody Johnson told the Carlton Club in London that the incident showed “this is not where [the British] want to be, over there, when they treat us like that - your prime minister”.

Given the gap between the European commission and Johnson’s government in the Brexit talks, sources in Brussels said the British prime minister would eventually need to shift on his red lines, and that it could be a tougher job if disdain for the EU among Tory MPs was fuelled.

The chair of the Bundestag’s foreign affairs committee, Norbert Röttgen, who is a senior member of Angela Merkel’s CDU party, tweeted: “Xavier Bettel’s speech yesterday did not serve the European cause. His public venting ignored that a deal is still in everyone’s interest.

“Even without a deal there will be a post-Brexit life, which means that right now everyone needs to behave in a way that avoids animosity.”

Play Video 2:20 Luxembourg PM mocks Boris Johnson after PM skips press conference – video

The EU and the UK have agreed to intensify negotiations, with the two sides due to meet daily rather than bi-weekly from next week.

Johnson spoke to the German chancellor on Tuesday, saying there was a need to “accelerate efforts to reach a deal without the backstop, which the UK parliament could support, and that we would work with energy and determination to achieve this ahead of Brexit on 31 October”, the prime minister’s spokesman said.

But there remain grave doubts in Brussels that the UK has an alternative to the Irish backstop that can both satisfy the EU and secure the support of a majority in the British parliament.

One senior EU diplomat said: “No one knows what the UK in the end wants, no one knows whether that’s acceptable to us and no one knows how he’s getting the numbers.”

In the light of what EU officials described as “wide of the mark” press reports suggesting there had been good progress in recent talks, there was some suspicion that Johnson was working to trap the EU in a “blame game”.

A diplomatic source said: “It was discourteous of Bettel, but a British prime minister as we once knew them would have braved it and probably even turned it to his advantage.

“The main problem is that this only reinforces the ‘them v us’ narrative that Johnson has used before, and whilst the continental press rejoices, it will not do him any harm. A leader of a bigger country would not have used such an important occasion to better his own profile.

Profile Who is Luxembourg’s prime minister, Xavier Bettel? Show Hide Xavier Bettel has been prime minister of Luxembourg since 2013. He was previously the mayor of Luxembourg City. Born in Luxembourg City in 1973, he studied law at university in both France and Greece, and combined working as a barrister with his political career. Bettel was first elected to municipal office in 1999, representing Luxembourg’s centre-right Democratic party. In 2018 he became the first openly gay prime minister in the world to be re-elected for a second term. He married his husband, Gauthier Destenay, in May 2015, five months after his government had legalised equal marriage. Photograph: Yves Herman/X00380

“On substance, I think it was also an answer to the increased spin from London about having a deal whilst there is still such a wide gap at the table.”

The chief spokesperson for Jean-Claude Juncker, the European commission president, sidestepped a question about Bettel’s decision to hold the press conference without Johnson, saying “[Juncker] felt that it was a very good meeting and that the atmosphere was very friendly and this is also what he conveyed to the press when he was doorstepped after the meeting. So this is how he perceived the meeting up until then.”

EU sources close to the talks paint a less dramatic picture behind closed doors, where Juncker; the chief Brexit negotiator, Michel Barnier, and his officials schooled Johnson in EU law on food, animal and plant health. EU insiders think the Luxembourg lunch neither advanced nor diminished the chances of a deal, but was a logical part of the process – a process where the endgame is still up in the air.

The meeting has raised expectations of an embryonic deal in the works, involving a rebranded Northern-Ireland only backstop, as the UK moves towards an all-Ireland food and agricultural zone. “There is the credible kernel of a deal that isn’t there yet, but could be there by mid-October,” an EU source said.

But others point to the enormous gulf between the two sides on numerous questions, above all whether Johnson could accept Northern Ireland remaining in a customs union with the EU.

Any revamped deal is also likely to include a less ambitious statement on future EU-UK relations, but it is unclear whether such a watered-down political declaration would command support of Labour MPs, if it seen as stripping away workers’ rights and environmental protections.

Johnson is due to talk to Merkel and the French president, Emmanuel Macron, about his latest Brexit proposals at the United Nations general assembly in New York next week. But there is no expectation in Brussels of written solutions being tabled until after the Conservative party conference the following weekend.