Emmanuel Macron may look like the nemesis of Brexiteers, but really he's their best friend.

In his letter to European citizens published Monday night, the French president wasted no time in giving Brexit leaders both barrels. "Who told the British people the truth about their post-Brexit future? Who spoke to them about losing access to the EU market? Who mentioned the risks to peace in Ireland of restoring the border?" he thundered. "Retreating into nationalism offers nothing; it is rejection without an alternative."

Stalwarts of the Remain camp such as Alastair Campbell, Tony Blair's former communications chief, praised Macron's message. "Suggests he has thought more deeply about Brexit than some British leaders," Campbell tweeted.

While Macron's diagnosis of the causes of Brexit (a failure to protect people from "the shocks of the modern world") is open to debate, it would be hard to argue that his suggested cure will go down well on British doorsteps. The French leader's grand plan, complete with a raft of new powers for the EU and a multitude of new agencies to exercise them, is just the kind of vision that has made many Britons skeptical of Brussels.

If campaigners such as Campbell achieve their aim of a second referendum, it's not difficult to imagine Brexiteers sending Macron's letter to every British household, perhaps emblazoned with the words "See? This is what we warned you about!"

At the end of his letter, Macron declares that in a reformed Europe, "the UK, I am sure, will find its true place."

Not all of Macron's vision will become reality. But his letter is a reminder that the European Union has already changed since Britain voted to leave — even while the U.K. remains a member. And many of those changes go against positions that British governments have long advocated. The bloc has embarked on greater defense cooperation, its leaders are calling for more trade protectionism and there is much talk of more state intervention, at both European and national level.

While Macron's missive makes Remainers' cause harder, he also hints at a vision for Britain outside the EU that may be more palatable to those anxious about Brexit. Macron acknowledges, for example, that the U.K. will remain a major player in the Continent's security and defense. He suggests establishing a European Security Council "with the UK on board." More broadly, he talks about a Europe that advances "sometimes at different speeds, and that is open to all."

That plays into another vision, outlined by Macron last year — of a Europe of concentric circles, in which members of the inner core integrate deeply while other countries subscribe to certain levels of cooperation and shared values. Not everyone buys into that concept. Germany is more skeptical about a multispeed EU, as are many Central and Eastern European states. But it already exists to some extent, with some EU members in the eurozone and passport-free Schengen area while others remain outside.

At the end of his letter, Macron declares that in a reformed Europe, "the UK, I am sure, will find its true place." The French president's grand plan makes it even more likely that that place will be outside the EU.

This article is part of POLITICO’s premium Brexit service for professionals: Brexit Pro. To test our our expert policy coverage of the implications and next steps per industry, email pro@politico.eu for a complimentary trial.