Ultimatum comes as sources say PM was ‘surprised’ by levels of checks on the border

Boris Johnson has been set a two-week deadline to table a plan for replacing the Irish backstop as further embarrassing details emerged of the prime minister’s chaotic visit to Luxembourg.

France’s president, Emmanuel Macron, and Finland’s prime minister, Antti Rinne, told reporters in Paris that they were both “concerned about what is happening in Britain”.

“We need to know what the UK is proposing,” said Rinne, whose country currently holds the EU’s rotating presidency. “Loose talk about proposals for negotiations is irresponsible … The UK should make its possible own proposals very soon if they would like them to be discussed.”

Rinne said: “We both agreed that it is now time for Boris Johnson to produce his own proposals in writing – if they exist. If no proposals are received by the end of September, then it’s over.”

A deadline of 30 September would be highly problematic for the prime minister as it falls on the eve of the Conservative party conference, and it remains to be seen whether the EU will stick to the threat.

Johnson would be wary of showing his hand at such a sensitive point given the potentially negative reaction by his party to any movement towards the EU’s demands on the backstop.

Rinne said that he intended to speak to the European council president, Donald Tusk, and Johnson to discuss the need for swift action from the UK.

EU leaders want a clear run for negotiations before a summit on 17 October so that they need not engage in detailed talks on the issue and can nod through any deal.

Johnson spoke to the European commission president, Jean-Claude Juncker, by phone on Wednesday afternoon. A Downing Street spokesperson said they discussed their continued determination to reach a deal.

They also discussed their lunch meeting in Luxembourg on Monday, which was described as “positive and constructive” during the call.

It emerged earlier in the day that during the lunch meeting Johnson had expressed surprise to his advisers when he was informed about the scale of checks that would still be needed on the island of Ireland under a plan the government has mooted for the Irish border.

The two-hour lunch preceded Johnson’s humiliation at the hands of Luxembourg’s prime minister, Xavier Bettel, when he failed to attend a press conference due to anti-Brexit protests.

EU officials said the advantage for them had been in being able to spell out the problems directly to the prime minister. “It seems to have helped the penny drop,” said one diplomatic source.

Will Corbyn’s Brexit referendum strategy work? Our panel responds | Polly Toynbee and others Read more

During talks with Juncker and the EU’s chief negotiator, Michel Barnier, the prime minister was shown in detail how allowing Northern Ireland to stick to common EU rules on food and livestock, known as sanitary and phytosanitary measures (SPS), would still fail to avoid checks on the vast majority of goods that cross the Irish border.

Downing Street has described as “nonsense” a report in the Financial Times that Johnson turned to his chief negotiator, David Frost, and the Brexit secretary, Stephen Barclay, and said: “So you’re telling me the SPS plan doesn’t solve the customs problem?”

But senior EU sources confirmed that Johnson had expressed surprise during the lunch at the complexity of the situation, and that it appeared to have been a “bit of a reality check to hear it from EU officials”.

Sources said it was not the case that Johnson had failed to understand the role of the shared customs territory in the Irish backstop but that it was the scale of checks that would still be necessary without such an arrangement that appeared to hit home.

A second EU diplomat confirmed: “When the commission explained the technical challenges and enduring need for customs checks under the UK proposals, Johnson expressed surprise in the direction of his advisers.”

In his address to the European parliament on Wednesday, Juncker hinted at the problems that remained. “I have no emotional attachment to the backstop,” he said of his talks with Johnson. “But I made clear that I do have an intimate commitment to its objectives.”

Given the wide gap between the two sides, Juncker also expressed doubts about the possibility that a mutually agreeable replacement for the Irish backstop would be agreed before 31 October when Johnson has said the UK will leave, “do or die”.

Q&A What is a Northern Ireland-only backstop? Show Hide The British government’s version of Brexit involves the UK ultimately leaving the single market and customs union, requiring the return of a range of checks on goods crossing the border between Northern Ireland and the Republic of Ireland. The “backstop” is intended as a standstill placeholder to ensure such checks do not have to be imposed between Brexit happening with a deal, and the start of a new free trade agreement yet to be negotiated between the UK and the EU. Theresa May's withdrawal agreement proposed keeping the whole of the UK in a shared customs territory with the EU during this period. An alternative idea involves only Northern Ireland staying in the EU’s customs territory. That would place a customs border in the Irish Sea. May described it as a threat to the constitutional integrity of the UK, but the new prime minister, Boris Johnson, has opened the current talks by proposing an all-Ireland agri-food zone. The suggestion is that he will seek to quietly build on that with further NI-only arrangements. Given an NI-only backstop was an EU proposal in the first place, the U-turn would be warmly welcomed in Brussels, although attempts to give the Northern Ireland assembly a veto on its continuation would not be acceptable, and the DUP would be unlikely to support the prime minister in such a move in parliament. If there is a no-deal Brexit, then there is no backstop. Daniel Boffey

The UK said after the Juncker lunch that it would now move to negotiating daily with the EU rather than twice a week but a spokeswoman for the European commission told reporters on Wednesday that there had been no such request as yet.

She said: “We have said and reconfirm that we are fully available to meet 24/7 seven days a week as soon as the UK would like to meet us. But for the moment I do not have any new announcements or scheduling to pass on to you today.”

Barnier had suggested that there was a lack of seriousness in the UK’s approach during his own statement to the European parliament.

“Almost three years after the British referendum, ladies and gentlemen, it is certainly not a question of pretending to negotiate,” he told MEPs.

Meanwhile, Johnson had his first conversation with the president of the European parliament, David Sassoli, on Wednesday. Sassoli is understood to have told the prime minister that any agreement would need to be agreed by both the Commons and the EU’s parliament, and “robust debate and parliamentary scrutiny is essential”, in a thinly veiled criticism of the decision to prorogue Westminster.