Brussels opposes Theresa May’s plan to delay Brexit until 30 June, according to a leaked internal EU diplomatic note.

The review of the Brexit situation drawn up by EU officials says national leaders will face a “binary” choice of a short Article 50 extension to before May 23, or a long delay to at least the end of this year.

“Any extension offered to the United Kingdom should either last until 23 May 2019 or should be significantly longer and require European elections,” the leaked document says.

“This is the only way of protecting the functioning of the EU institutions and their ability to take decisions.”

The document, drawn up for European Commissioners, says that "any other option (as for example an extension until 30 June 2019) would entail serious legal and political risks for the European Union and would import some of the current uncertainties in the United Kingdom into the EU27".

It adds that "any other scenario would also have direct legal and practical consequences for the election of Members to the European Parliament in 14 of our Member States".

On Wednesday afternoon Theresa May wrote a letter to European Council president Donald Tusk formally requesting an extension past next Friday, the day Britain is currently set to leave the EU. She said she was “not prepared to delay Brexit any further than the 30 June”.

The decision of whether to extend and by how long is not up to Brussels – but the 27 other EU member states, who must unanimously approve any delay. Though there are differing views in EU capitals about a delay, throughout Brexit talks member states have however so far stuck close to the European Commission’s line.

If the Commission’s advice is followed by leaders on Thursday, Theresa May will have to choose between a very short extension, or a longer one. The latter is likely to be unpalatable to her party and Cabinet, while the former may not be long enough to achieve anything in – depending on what her plan is.

The reason for the concerns laid out in the document, which was reviewed by the Commission at its weekly meeting on Wednesday, is because of the logistical effect such a middling-length extension could have on the EU.

EU member states are due to get extra MEPs in the coming European Parliament elections because of Britain’s departure – with the UK’s old seats split up between the other countries. As a result, the EU says it would need to know by late April so that seats could be allocated properly ahead of the elections.

The diplomatic note also says that any long extension should see Britain, “in a spirit of loyal cooperation”, commit to “constructive abstention” on key issues, such as the budget and selection of Commissioners.

Britain Before Brexit: Northern Ireland Show all 12 1 /12 Britain Before Brexit: Northern Ireland Britain Before Brexit: Northern Ireland Derry, Londonderry A garage door displaying unionism, bolted shut, like a visual representation of Brexit Britain, locked to outsiders, safeguarding what’s inside Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry Rossville Street, the site of Bloody Sunday, where messages demand a severance with England. From this perspective, Britain is England in sheep’s clothing, the real empire, the centre of colonial power Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Bangor A political message in paint not yet dry, still forming, setting, adjusting, or in old paint finally eroding, melting away Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Bangor Moral judgement frames a residential view. The message seeks to make everybody involved in the religious narrative: those who don’t believe are those most in debt Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Castlerock The beach is sparse and almost empty, but covered in footprints. The shower is designed to wash off sand, and a mysterious border cuts a divide through the same sand Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast Two attempts to affect and care for the body. One stimulated by vanity and social norms and narratives of beauty, the other by a need to keep warm in the winter night Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast The gate to an unclaimed piece of land, where nothing is being built, where no project is in the making, where a sign demands the creation of something new Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry Under a motorway bridge a woman’s face stares, auburn and red-lipped, her skin tattooed with support for the IRA and a message of hostility to advocates of the Social Investment Fund Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry The Fountain Murals, where the curbs and the lampposts are painted the red, white, and blue of the Union Flag. A boy walks past in the same colours, fitting the scene, camouflaged Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Coleraine A public slandering by the football fields, for all to see or ignore. I wonder if it’s for the police or for the community Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast A tattoo parlour, where the artist has downed tools, momentarily, bringing poise to the scene, which looks like a place of mourning, not a site of creation Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry A barrier of grey protects the contents of this shop, guarding it from the streets outside, but it cannot conceal it completely, and the colours of lust and desire and temptation cut through Richard Morgan/The Independent

On Tuesday night EU chief negotiator Michel Barnier warned: “I am here to ask a very legitimate question on behalf of the European authorities – which is why do you want an extension? What for? What’s the objective of an extension? What use would it be? An extension has to be useful.”

Mr Barnier warned that “extending uncertainty without a clear plan” would have political and economic costs for the EU, and that a longer extension needed to be linked to something – such as a new political process in the UK to find a solution to the deadlock.