It is now too late for any Brexit deal Boris Johnson strikes with Brussels to be formally approved by leaders at this week's summit, EU member states have warned.

The news is a blow for the prime minister, who had hoped to be able to put a final agreement to MPs at a sitting of parliament on Saturday after coming back from the summit with a final deal.

European capitals say they need more time to properly look at a deal – assuming negotiators can strike one ahead of the meeting at all. Leaders are at best expected to give the deal "political" approval and words of encouragement – which may not be enough for MPs back in Westminster.

"I cannot imagine leaders tomorrow being able to say anything more than 'this doesn't look too bad, let's continue to work with the UK to finalise the details'," a senior EU diplomat said on Wednesday, less than 24 hours ahead of the meeting.

"It's clear that there can only be political agreement tomorrow and Friday because we haven't seen any texts: we can't say yes without having done legal scrubbing. It can only be a political yes, we need much more time.

The state of affairs recalls warnings from member states in previous days, who had said an agreement needed to be finalised well ahead of the summit so their teams could scrutinise it.

Member states have been kept mostly in the dark about the details of talks so far, with the Commission's team in a "tunnel" with the British side.

"I have no idea where we stand at all because we just don't know. We'll have to see what we get from Michel Barnier at 2pm this afternoon," one diplomat said.

"The more the deal is different from what we had prepared early 2018, the more time it will take for legal experts and customs experts to judge. The closer it is to what we had prepared a year and a half ago the easier it will be.

"But it's now clear that is too late to give a formal yes at this European Council. There's just not enough time. We don't need that, we can only give a political yes and come back to it another time."

It remains unclear whether even a deal between the deals will be possible at all, with conflicting reports about how talks are progressing. The 2pm ambassadors' meeting referenced by the EU diplomat has now been put back to 5pm to allow more time for talks.

It's clear that there can only be political agreement tomorrow and Friday because we haven't seen any texts EU diplomat

“The teams worked into the night and continue to make progress. They will meet again this morning," one groggy UK official told reporters in Brussels. Negotiations in the Commission's Berlaymont headquarters ran past 1am on Wednesday morning and picked up again at 9am the same day.

Back in Westminster, the prime minister's official spokesperson said: “The talks overnight were constructive but there remains work still to do. There certainly do remain issues to resolve.”

Asked if there had been any progress overnight, he said: "It is a fair summary to say that progress was made but also that there is more work to do.”

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Cabinet, which was delayed from Tuesday because of the Brexit talks, is not due to meet at 10 Downing Street at around 2.30pm on Wednesday. The meeting had earlier been expected to take place at 4pm, and the spokesman said the change was due to diary issues.

Mr Johnson was meeting with representatives of the DUP in Number 10, and his team were also due to continue meetings with Conservative MPs, including members of the eurosceptic European Research Group.

EU leaders will start arriving in Brussels for this week's summit after lunch on Thursday. The meeting will continue into Friday afternoon, with other items on the agenda including the EU budget, enlargement of the union, and a common response to Turkish military action in northern Syria.