Having been apparently dead two weeks ago, a Brexit deal has suddenly risen Lazarus-like from the dead. However, the prospect of a last-minute deal raises significant difficult issues for a number of actors in the Brexit drama.

This miraculous resurrection was made possible by a sudden and dramatic shift in the policy of Boris Johnson’s government which abruptly abandoned many of its key red lines and has signed up to a deal that, to paraphrase Seamus Mallon’s take on the Good Friday Agreement, amounts to the Theresa May deal for slow learners.

Indeed, the only really significant losers are the DUP who end up with arrangements that leave Northern Ireland technically within the UK customs zone but effectively outside of it.

The striking of a deal at this last minute raises a host of significant legal and political issues. It will be necessary to put legal form on what are very complicated and intricate arrangements. In addition, the deal will have to be ratified by the European Parliament and a large amount of legislation will have to be passed in Westminster. It is unlikely to be possible to do this by October 31st so some kind of extension of UK membership under article 50 will probably still be necessary.

The question then becomes how long that delay should be. Johnson may want only a short extension that gives only the time needed to ratify the deal. This would put parliament in a difficult position. If the extension was short and parliament rejected the legislation implementing the deal, it would risk the UK crashing out with no deal.

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Moreover, many in the opposition favour remaining in the EU or a much softer Brexit than that envisaged by Johnson. They would want a longer extension that would allow the UK to hold a second referendum before any general election.

Indeed, many MPs will be rightly reluctant to approve a deal in principle when the precise legal details of how it will apply have yet to be set out. Under the Benn Act, once parliament approves the agreement in principle then the obligation on the prime minister to seek an extension falls away, thus removing the main source of parliament’s current leverage over Johnson.

For the Irish Government and the EU, the speed and magnitude of the prime minister’s change of heart should also raise suspicions. The EU will be aware that this is their moment of maximum leverage over Johnson. The passage of the Benn Act has prevented him from leaving without a deal, he has no majority in parliament and will soon have to face a general election.

The British prime minister appears to have suddenly woken up to the fact that if he did not make major concessions he was facing a long extension and possibly, in the end, no Brexit at all. Such an outcome would bring electoral catastrophe for the Tories. However, if Johnson can achieve the UK’s exit by the time he faces the electorate it will be much easier for him to hoover up Brexit Party support.

More importantly from the EU perspective, once Brexit takes place, it is legally very difficult to reverse. While extending UK membership before Brexit only requires a vote in the European Council, bringing the UK back in once it has left means following the very laborious accession procedure set out in article 49 of the Treaty on European Union which includes ratification by the parliament of each member state.

This means that once Brexit has taken place, the EU will lose much of its leverage over Johnson. The union will accordingly be very keen to ensure that the detail of how the deal will function is nailed down with great precision in order to avoid a scenario where, having achieved the UK’s exit from the EU, Johnson then begins to seek to wiggle out of the commitments he has given.

Brexiteers argued for years that the EU only makes concessions at the last minute. Yet what has happened is that it is the UK, at the very last minute, has dramatically dropped most of its key demands. It is a measure of the prime minister’s evident desperation to get a deal that the UK now appears to be willing to sign up to internationally unprecedented measures that place the economic policy of a part of its territory effectively under the control of another polity.

However, EU negotiators and governments are no doubt aware, this desperation in part reflects the fact that legal and political realities mean that if Johnson can get Brexit over the line their ability to bend him to their will and hold him to his promises will be significantly reduced. A last-minute deal in principle without legal specifics is welcome from their perspective but they will need to be careful to ensure that the rush to get things done does not cause them to drop their guard.

Ronan McCrea is professor of constitutional and European law at University College London