Brexit has now happened. Yet Brexit, in any meaningful way, is unlikely ever to happen.

As a legal act, Britain's divorce from its 27 neighbours and key trading partners is now under way. On Wednesday, Prime Minister Theresa May served notice of separation from the European Union.

As a practical outcome, however, Brexit will be very unlikely to deliver any of the things its supporters believed they had voted for.

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Of the 17 million Britons who voted to advise their government to leave Europe in last year's referendum, the largest group told pollsters that they did so primarily because they wanted less immigration from Europe. (Leaving the EU would not affect refugee flows or immigration from Britain's former colonies.) Smaller groups said they wanted less trade with Europe, an end to European regulations and an end to payment of dues to Brussels; many said all four.

The Brexit ahead: A guide to the issues and people involved

Even if Ms. May's pro-Brexit government stays intact through the two years of exit negotiations, the real meaning of Brexit will not be defined until Britain negotiates and signs a post-Brexit agreement with its neighbours. This will take years (the Canada-EU trade deal took more than a decade and is still not ratified): Angela Merkel, the German Chancellor, made it very clear that Britain may not begin negotiating its future relationship until exit negotiations are finished in 2019. And the proposed resolution the European Parliament will vote on next week forbids Britain from negotiating free trade with any non-EU countries until it has left the union two years from now (or else it will lose its right to negotiate a deal with Europe).

Britain will be negotiating from a position of weakness: As the EU Parliament makes clear in its resolution, any deal will require the unanimous approval of all 27 member-country legislatures plus a majority vote in the EU parliament. A few of those countries rely heavily on trade with Britain. Most do not, and all want to set a harsh example to discourage future departures.

There is a vaguely articulated belief among ardent Brexit proponents that Britain might be able to maintain and even expand its economy without EU trade, by building trade relations with China, Canada, the United States and other advanced countries.

That belief was acknowledged by Theresa May when she declared, in her speech to parliament Wednesday, that post-Brexit Britain would be a "great, global trading nation."

It could not. A study this month by the Centre for Cities found that 61 of Britain's 62 largest cities rely on the EU more than any other market for their employment and earnings; even the least EU-dependent cities sell more than a quarter of their exports to Europe.

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The EU accounts for 46 per cent of Britain's non-agricultural exports. Canada, China, India, Mexico, Singapore and South Korea together make up only 10 per cent. They're valuable, but no replacement.

Britain is also home to hundreds of factories owned by Asian firms choosing to manufacture cars, electronics and appliances in Britain so they can have access to the 500 million consumers of Europe without paying steep (typically 15 per cent) import tariffs. These factories, such as the sprawling Nissan car plant in Sunderland, are among the country's largest employers. Without free trade with the EU, they would have no reason to remain located in Britain. Nor, for that matter, would many British manufacturers.

So Britain will have no choice but to negotiate a post-Brexit free-trade deal with the EU (individual EU member countries have no trade-negotiating authority). And the EU will insist that access to its markets will only be granted in combination with its other core values of free movement and regulatory compliance.

After years of negotiations, post-Brexit Britain will hope to end up like Norway, an economically successful country that's not part of the EU. Norway's financial success depends on its access to the EU's common market. And in exchange for that access, Norway must allow free movement of EU citizens across its borders; Norway must comply with all EU business laws and regulations; and Norway must pay substantial dues to the EU (without having any influence over its rules).

That, in the end, is what Brexit will most likely look like. Whether Britons will be willing to call it Brexit is another question.