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EU diplomats have been told it will be impossible to extend the Brexit transition after 1 July 2020, if Boris Johnson decides in the second half of next year he wants extra time to negotiate the future relationship.

At a private meeting on Tuesday, diplomats from the 27 member states were told by senior EU lawyers that 1 July 2020 was a hard deadline that could not be changed, if no extension was agreed in the first half of the year.

The UK is due to leave the EU on 31 January and then falls into an 11-month transition period, where it will remain in the EU single market and customs union without voting rights.

Under the withdrawal agreement negotiated by Theresa May, the British government has the option to request a one-off extension of the transition of one or two years, before 1 July 2020.

The prime minister has promised to legislate against any extension to the transition, but that has not killed speculation that the UK may seek a last-minute delay in late 2020 if a no-deal exit is looming.

Lawyers at the EU council’s legal service moved to quash that option on Tuesday, by delivering an uncompromising message that the transition would cease to exist after 1 July 2020, because the withdrawal agreement leaves no provision for later agreement.

The message was conveyed in stark terms by EU lawyers, who want this point transmitted to all EU27 national capitals.

However, the EU has a track record in declaring immovable deadlines, only to find a workaround when a no-deal deadline loomed. Some EU officials had previously insisted the UK would have to leave the EU before European parliamentary elections in May 2019, only for the UK to secure a delay and elect British MEPs.

EU diplomats believe Boris Johnson when he says he doesn’t want to extend the transition period. “It is probably one of the areas where Johnson isn’t lying,” quipped one person in Tuesday’s meeting, a joke appreciated by others.

The European commission, which will handle day-to-day negotiations with the UK, has promised to draft a comprehensive mandate for the talks in early February, soon after the UK leaves.

That will be a document setting out the EU red lines on the future relationship covering trade, security, foreign policy, education and research and other areas.

“We will organise these negotiations to make the most out of the short period. On 1 February we will be ready to propose a mandate for the negotiations,” the European commission president, Ursula von der Leyen, told MEPs this morning.

But the commission is already warning member states not everything can be done in 11 months. Von der Leyen has said the commission will have to prioritise key topics, expected to be free trade in goods in exchange for British guarantees of fair competition, as well as fisheries.

EU officials continue to stress the UK parliament must ratify the withdrawal agreement. The Guardian understands that the European parliament has pencilled in 29 January for ratification of the Brexit deal, assuming the process goes smoothly at Westminster.