Dear European friends,

We are fast approaching a critical period in Britain’s Brexit drama. Incredible though it may seem, there is now a serious chance of the British voting in a second referendum to stay in the European Union. What an extraordinary boost that would be to the whole post-1945 project of building a better Europe!

To get there, we need a little help from our friends. I can well understand why many Europeans just want to get the whole thing over and usher the UK out of the door. We have now had 900 days of the British government either not being able to articulate a negotiating position, or making unrealistic demands, or, as this week, being unable to deliver parliamentary approval for the deal it has negotiated.

As the clock ticks down towards B-day, 29 March 2019 – bringing the danger of a chaotic no-deal Brexit – I can see why, at the European council in Brussels this week, leaders such as the German chancellor, Angela Merkel, might want to help the British government just to “get over the line”. For then the EU could get back to confronting all its other major challenges, including populism and gilets jaunes (yellow vests), the continued problems of the Eurozone, immigration, Vladimir Putin and Donald Trump.

But that would be short-sighted. Yes, in the short term, helping the British government over the Brexit line would bring some certainty, enabling the EU to get back to other business. But in the long term Brexit would create a festering British ulcer, hurting and weakening the body of the European Union.

The ulceration would begin soon after B-day. Britain would then have to negotiate its actual future relationship with the EU, on the basis of a vague and non-binding political declaration, and from an exceptionally weak negotiating position. Those negotiations would take years, and be very difficult. The false promises of the Brexiteers would soon be exposed. To avoid taking the blame themselves, the Brexiteers and Britain’s Eurosceptic press (no sticklers for truth, to put it mildly) would blame the country’s misfortunes on “the Europeans”, and especially on the French – something the English have been practising for 700 years.

Even if we were to get beyond this toxic blame-game, it is a dangerous illusion to believe that Britain would go on happily, constructively cooperating with the rest of Europe on foreign policy, defence, counter-terrorism (one thinks of the latest victims in Strasbourg) and intelligence-sharing and all the other areas in which the UK substantially contributes to Europe, while being mired in unhappiness about the rest of the relationship. That’s not how politics works, especially in an age of populism. It is also a delusion to think that within a few years the British would come back, with their tails between their legs, begging to rejoin. That’s a serious misreading of the British character. In short, there would be a dynamic of divergence, not of convergence.

Of course we might lose the referendum vote again. Even then, the country would be in no worse a position than it is now

So the only good Brexit is no Brexit. The chances of that – what May charmingly calls the “risk of no Brexit” – have soared in recent weeks. Whereas elsewhere nationalist populism has seriously impaired the workings of democracy, in Britain it is working. I have spent a lot of time with British MPs recently, and seen how seriously they are treating their role as elected representatives in a critical moment. As a result, the mother of parliaments is now taking back control. Nobody knows what will emerge from its often arcane and operatic procedures. A new election? A government of national unity? A vote for Norway plus (British membership in the EFTA pillar of the EEA, plus a customs union)? No deal by accident rather than design? Anything is possible. Nothing is certain – except that there will be several more weeks of fireworks, smoke and confusion. But the option quietly gaining support among MPs is a second referendum.

It is ridiculous to suggest it would be undemocratic for Britain’s sovereign parliament to take the question back to the people. The resulting referendum campaign could be angry and divisive. But one has to weigh that short-term risk against the much larger long-term risks to both Britain and Europe. A referendum would be short-term pain for long-term gain.

Of course we might lose the referendum vote again. Even then, the country would be in no worse a position than it is now, and arguably in a better one: no one could then argue that people did not know what they were voting for. What is certainly true is that, to ensure a Britain that constructively re-engages, helping to bring the EU the reforms it badly needs, we need to achieve a decisive margin for staying in. Recent opinion polls are pointing in that direction, but we must make a much better pro-European campaign than we had in 2016.

If and when the parliamentary vote for a second referendum comes, we will need just a few things from our EU partners. Article 50 would have to be extended for a few months. A report by constitutional experts at University College London suggests a referendum could be done properly in 24 weeks. That points to an article 50 extension into the summer of 2019, which would entail addressing a tricky issue around British participation in the European elections in May. The ruling this week of the European court of justice establishes that, following a vote to remain, the UK could unilaterally revoke article 50 and stay as a member of the EU on its current terms. Apart from that, all we need from you, my friends, is a clear, simple, positive message, without ifs or buts: “We want you to stay!”

If you are persuaded that building a stronger Europe in a dangerous world needs the UK to be pulling its weight inside the EU; if you regard any blow struck against the dark forces of nationalist populism as a good thing; if what the UK has contributed to Europe over the centuries means anything to you; if you place any value on what Britain did for European freedom in the second world war, for postwar reconstruction and for helping to throw off the yoke of communist dictatorships; then let us have this chance. If you have worked with British colleagues, spent time at a British university, enjoyed some aspect of British sport or culture, or have British friends; if anything the British have ever done has touched your heart; then give us your solidarity and support. In helping Britain you will also be helping Europe.

Thank you.

• Timothy Garton Ash is a historian, political writer and Guardian columnist