Even if Theresa May gets her withdrawal agreement through parliament, Brexit is far from assured

Before we can depart the bloc, the government must pass an implementation bill which could be far more toxic than the deal we have seen so far

So, last week Theresa May agreed another new Brexit date with EU leaders. It delays the UK’s departure until 31 October. Whether or not this proves to be a trick or treat for the prime minister is unclear.

The reality is that even if this “flextension” is enough for May to get MPs to finally back her withdrawal agreement, the PM’s biggest challenge is yet to come – and her Brexit nightmare could continue long after Halloween has passed.

Unfortunately for May, overturning historic defeats on withdrawal by breaking parliament’s current deadlock is possibly the easy part.

May must ultimately win support for an EU withdrawal agreement implementation bill that has not yet been seen by MPs. This legislation is required to implement the terms of withdrawal. If it isn’t passed, no Brexit happens even if May can finally get support for the deal that we know about.

Angry disagreement over whether to support a customs union or the Irish backstop pale in comparison with what is understood to be in the implementation bill.

For one thing it would formally approve the £39bn divorce bill, which could still rise depending on how long Britain is stuck in a transition period agreeing a new trade agreement still to follow. This figure is likely to come under intense scrutiny by friends and foes of Brexit alike.

What’s more, if we are to have a transition period after we initially leave the EU, parliament must legislate for the temporary supremacy of EU law during the entirety of this process.

According to the withdrawal agreement, this process could continue potentially far beyond 2020 – as originally agreed between May and the EU, it could run until 31 December 2099. It is non-negotiable insofar as we would be bound by EU law during that period – it is contained in the implementation bill too.

This leaves May sitting on a ticking political time bomb whose flames could turn her plans for Brexit to ash.

It is far from clear she can get MPs to eventually support her deal, but even if she could, imagine the enormity of the task she faces next. In order to make good on promises made in a referendum to take back control from EU, MPs must vote to pay billions to the EU, entrench EU law into our legal system for possibly more than half a century and keep the UK under the jurisdiction of the European Court of Justice.

If the withdrawal agreement isn’t too politically toxic for May’s pro-Brexit backbenchers, the required terms of implementing the agreement surely are. Yet both must be approved for Brexit to happen and before any talks over future trade deals can start.

So what next for the prime minister? She might now want to stand down, call a snap election or consider holding a public vote. However, none of these options makes Brexit any easier to achieve. In fact, it’s never looked more unlikely and difficult.

Thom Brooks is dean of Durham Law School and advised the Electoral Commission on the EU referendum