The promise of the Leave campaign was to “take back control”. After decades of subservience to Brussels, Britain would regain sovereignty over its borders, money and laws. The 23 June 2016 was, Nigel Farage proclaimed, “independence day”.

Nearly three years later, the EU has never enjoyed more control over the UK’s fate. Nine days before Britain’s scheduled departure date, and faced with the threat of a no-deal Brexit, Theresa May has been forced to humiliatingly plead for an extension to Article 50. Such a demand can only be met with the unanimous approval of the 27 other EU member states. In other words, the UK’s future depends upon the whims of governments including Viktor Orbán’s Hungary and Matteo Salvini’s Italy. Far from taking back control, Britain has given it away.

In truth, it did so long ago. The moment May triggered Article 50 on 29 March 2017, the illusion of sovereignty conferred by the referendum vanished. When the EU drew up divorce proceedings it did so with the intention of maximising control. The withdrawal deal that Britain eventually reached required the approval of at least 72 per cent of member states, representing 65 per cent of the EU’s population.

May sought to maintain authority by threatening to leave with no agreement – but neither she, nor Brussels, ever regarded this masochistic option as credible. As a consequence, the UK was forced to capitulate on multiple fronts: accepting no advance trade deal, agreeing to pay a £39bn divorce bill and issuing a unilateral guarantee of EU citizens’ rights. In order to prevent a hard Irish border, the UK would sacrifice sovereignty by agreeing to a “backstop” – de facto customs union membership – from which it has no unilateral right to withdraw.

Britain, it has transpired, cannot have its cake and eat it. But the irony is that it was already doing so. The UK enjoyed opt-outs from the euro (the only member state other than Denmark to do so) and the borderless Schengen Zone, and received a £4.9bn budget rebate.

True, the EU did not grant the UK greater control of free movement (partly paving the way for Brexit). But David Cameron was awarded further concessions during his renegotiation of the UK’s membership. They included an official exemption from “ever closer union”, a four-year ban on in-work benefits for EU migrants (activable for seven years) and greater safeguards for the City of London.

In the instance of free movement, the UK has long had the flexibility to impose greater control. Though EU citizens are initially permitted to live in any member state, after three months they must prove that they are working (employed or self-employed), a registered student or have “sufficient resources” (savings or a pension) to support themselves and not be “a burden on the benefits system”. Far from being unconditional, then, the right to free movement is highly qualified.

Yet the supposedly immigration-averse UK has never enforced these conditions. Even under May, the Home Office deemed the cost of recording entry and exit dates too high. Since most EU migrants are employed (and contribute significantly more in taxes than they do in benefits), there was no economic incentive to do so. And as cabinet ministers have learned, it is easier to make a case for reducing immigration in theory than in practice. Which migrants can the UK do without? The honest answer is invariably few.

For the UK, Brexit has been a lesson in the realities of sovereignty. While Britain – one against 27 – has been routinely humiliated, Ireland, its former colony, has been empowered. The EU can erode national sovereignty – as it did during its immiseration of Greece – but it can also amplify it.

Theresa May, an inadequate and enfeebled prime minister, has failed to take back control. But as the UK’s crisis deepens it will become ever clearer that no one meaningfully could.