EU referendum: With four days until the UK votes, Brussels prepares for the unthinkable outcome of Brexit

No country has ever pulled out of the EU before, and the UK's departure would be a devastating blow to an already stumbling European project

Brexit could mean Britain is suspended from EU negotiations while it prepares to leave (Rex)

For weeks, and urgently for several days, Brussels has been thinking the unthinkable. For six decades, the European Union has expanded – from six nations to 9, 10, 12, 27 and now 28. No country has ever pulled out before. Or, almost no country. Greenland opted out of its membership as a Danish colony in the 1980s, but hardly anyone noticed.

The departure of Britain – the third-largest economy in Europe, the fifth-largest in the world – would be a devastating blow to an already stumbling European project. If Britons vote to leave, the first reaction of Brussels and EU governments will be that of any jilted partner: “How do we protect ourselves?”

Senior European politicians and officials have been holding confidential talks for several weeks on a so-called Plan B for Brexit. The central focus of the discussion has not been how to smooth Britain's departure, or how to make Brexit a success – it has been how to make Brexit so painful for Britain that other countries are not tempted to follow.

There will first be a co-ordinated EU reaction for public consumption along the lines of “au revoir, sorry to see you go but we can carry on perfectly well without you”. This will take the form of statements and maybe even new projects to suggest the EU will survive, or even thrive, without Britain.

Less publicly, there will be a determination to prove that abandoning the EU will have serious consequences. One senior French politician told The Independent: “The British like presenting retreats as victories. It is not in the interest of the other 27 to make this into another Dunkirk. It should be more like Napoleon’s retreat from Moscow.”

A leading European politician who is part of the Plan B talks told the Financial Times: “Making Brexit a success will be the end of the EU. It cannot happen.”

But what exactly that means is uncertain. The EU, whatever the Brexiteers may say, is not a monolith. It will inevitably be divided, with some countries and voices wishing to protect their trade relations with the UK by advocating an amicable separation and a friendly divorce. However, the two most influential politicians in the EU – Chancellor Angela Merkel and President François Hollande – are minded to make Britain suffer. They see this as an act not of revenge but of self-preservation.

France and Germany are working on draft guidelines on how they think the 27 other nations should react. Brussels officials say this could include moves to, in effect, freeze Britain out of the EU side of post-Brexit negotiations.

David Cameron will join the leaders of the other 27 countries for a summit in Brussels on 28 June, five days after the referendum. If Leave wins, he is likely to trigger Article 50 of the EU treaty, which provides for countries to negotiate their departure.

Hardliners in Brussels, Berlin and Paris suggest Mr Cameron should then be asked to leave the room. Alternatively, the 27 will call a second summit of their own to which Britain will not be invited. The EU treaty provides for a two-year period of negotiation before a member state leaves. A problem arises. Would Britain still be welcome to take part in EU decision-making while it is waiting to depart?

According to French sources, the Plan B discussions in Brussels and EU capitals have looked at the possibility that Britain would be, in effect, suspended from internal EU negotiations on all subjects, including Brexit. The UK would become rather like a candidate-member in reverse. It would not be allowed to take part in summits or councils of ministers which discussed the long and wearisome process of unwinding laws on everything from UK-EU trade to fisheries policy.

“One of the problems for the Plan B discussions is that we don’t know what, or who, we will be dealing with post a Brexit vote,” a French diplomat said. “Will it still be David Cameron? Or will it be Boris Johnson? Will Britain want to remain in the single market without being in the EU, like Norway. Or will it want to leave completely?”

The Brexit camp leadership now speaks of pulling Britain out of the single market (largely the creation of Margaret Thatcher and a British EU commissioner, Lord Cockfield.) Any other approach, the Brexiteers have been obliged to admit, would still commit the UK to free movement of people, to EU regulation and to contributions to the EU budget. However, a Brexit vote next Thursday would leave that vital question open for the House of Commons to decide.

Brussels officials say that negotiations on a partial Brexit – in which Britain remained part of the single market – might be easier and more amicable. They say, however, that they are working on the assumption that Britain will want to abandon all its legal and financial commitments to the EU.

The Plan B talks have also looked at what an EU, shorn of one of its most powerful but awkward members, might look like. There is talk of the announcement of a new, integrated EU policy for defence and security – especially security against terrorism. There are also voices pushing for the announcement of a “Great Leap Forward” in which a Britless EU would reaffirm its belief in further European integration and progress towards a “single European economic government”.

Other voices protest this would be a disastrous misreading of the Eurosceptic mood of all EU peoples, not just Britons. Both Jeroen Dijsselbloem, the Dutch politician who is head of the Eurogroup of Eurozone finance ministers, and Hubert Vedrine, a former French foreign minister, have suggested the EU response should be more realistic and more practical. Mr Dijsselbloem said: “The EU has moved ahead in leaps, but we haven’t always completed everything. That is our main task: to finish what we have started and show results to our citizens.”

Mr Vedrine suggested the EU should abandon its federalist commitment to an “ever closer union” and move towards more limited, practical joint policies on trade, free movement, open borders and the environment.

That sounds suspiciously like the kind of EU that Britain might have found easier to live with.

The EU referendum debate has so far been characterised by bias, distortion and exaggeration. So until 23 June we we’re running a series of question and answer features that explain the most important issues in a detailed, dispassionate way to help inform your decision.

What is Brexit and why are we having an EU referendum?

Does the UK need to take more control of its sovereignty?

Could the UK media swing the EU referendum one way or another?

Will the UK benefit from being released from EU laws?

Will we gain or lose rights by leaving the European Union?

Will Brexit mean that Europeans have to leave the UK?

Will leaving the EU lead to the break-up of the UK?

What will happen to immigration if there's Brexit?

Will Brexit make the UK more or less safe?

Will the UK benefit from being released from EU laws?

Will leaving the EU save taxpayers money and mean more money for the NHS?

What will Brexit mean for British tourists booking holidays in the EU?

Will Brexit help or damage the environment?

Will Brexit mean that Europeans have to leave the UK?

What will Brexit mean for British expats in Europe?