On Wednesday, Andrew Adonis, a leading anti-Brexit champion, appeared to put the strangest statement on his Facebook page. “I am deeply sorry for off-the-cuff comments I made during a live LBC radio phone-in last September,” he wrote. “I encourage all voters, whatever their position on Brexit, to vote Labour in the upcoming European parliament elections.” He then began a very uncharacteristic officious policy sermon including the statement: “Labour has put forward a sensible alternative plan that would ensure a close economic relationship with the EU after Brexit.”

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The first part is easy to explain. In September, Adonis had been asked who Labour voters should vote for if they wanted to see Brexit happen. He said they shouldn’t vote Labour. It was awkward. Given that Adonis is now newly announced as a Labour MEP candidate and pro-Brexit social media channels had begun circulating the clip of the discussion, it was appropriate to apologise for that. However, how to explain the second part?

Screenshots of the Facebook post circulated like wildfire on Twitter during Wednesday evening. Remain supporters were incredulous. Had he been hacked? Is this the really Andrew Adonis who overtly supported revoking article 50? The man who has toured the country relentlessly advocating remain? How can he now state there should only be a people’s vote if “a new customs union and dynamic alignment on rights and standards” cannot be secured?

Speculation quickly focused on the notion that Adonis had been handed an ultimatum to push that line, or be axed from the MEP candidate list. Remainers were horrified that Jeremy Corbyn’s office may have forced this on a prominent remainer they had already selected – and that Adonis may have actually agreed to it. As followers bombarded Adonis on Twitter, Facebook and email asking for urgent clarification, others in the Twitter political playground began making mischief.

Tory MP James Cleverly wrote: “If you read the first letter, of the second word, on every third line of this Facebook post a coded message says “Help me, Seumas is making me write this Brexit stuff that I don’t believe.”” Many others were also quick to suggest a role for Seumas Milne, Corbyn’s pro-Brexit director of strategy and communications, who is widely seen as the man guiding Corbyn: “reads like a hostage statement dictated by Seumas Milne” wrote Times columnist Iain Martin.

A full 24 hours after his Facebook post, Adonis explained on Twitter: “My statement yesterday is Labour’s approved conference policy. It doesn’t change my commitment to a public vote on any Brexit deal.” Few were appeased. Throughout Thursday, Adonis lost 2,300 Twitter followers. Though “only” 2% of his followership, it’s the highly active followership on Twitter that counts. The big pro-EU beast on Twitter has seen his engagement rate drop heavily and replies to his tweets are dominated by admonishments. The remain community is clearly in no mood to forgive. Unlike in 2017, the remainers are under no existential obligation to back Labour tactically in the 2019 European elections. After two years of being strung along with ambiguity, many would eagerly punish Labour.

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The Labour leadership is pushing opportunity into the jaws of defeat, given its overwhelmingly pro-European base. The international commission of Labour’s national policy forum, which includes trade unionists, MPs and constituency reps, has just voted unanimously that Labour’s EU election manifesto should pledge to hold a confirmatory referendum. On the same day, the Huffington Post obtained a draft Labour leaflet for the European elections saying that Labour would go ahead with Brexit and seek “a better deal with Europe”, with no mention of a confirmatory vote. It reported that neither Keir Starmer nor the head of the Labour MEPs, Richard Corbett, were consulted on the leaflet’s content. If that is the case, it is plain outrageous.

Whatever you think of Andrew Adonis’s unhappy moment, it is the secretive and dictatorial clique within Corbyn’s office that has brought party policy to breaking point with its base. If there is anyone who needs to do a volte-face, it is the Labour leadership – and right now.

• Dr Mike Galsworthy is director of the campaigns Scientists for EU and NHS for a People’s Vote