Theresa May has been defending her Brexit plan, after it was heavily criticised last week. FactCheck examines two of the main concerns.

Under the proposals, the UK will enter a “transition period”, after it formally leaves the EU in March 2019. Both sides will then use their “best endeavours” to reach a follow-up Brexit deal.

However, some MPs are worried about what might happen if that follow-up deal doesn’t materialise. But do their criticisms stack up?

The ‘threat to the integrity of the United Kingdom’

If a future deal is not reached, Theresa May’s proposal provides a safety net, known as the “backstop” plan.

It’s designed partly to prevent a physical border being introduced between Northern Ireland and the Republic of Ireland (which is part of the EU).

Most politicians agree there should not be a “hard border”. But, controversially, the backstop plan means Northern Ireland could be treated differently from the rest of the UK.

In particular, Northern Ireland would have deeper ties to the EU on trade and customs than England, Scotland or Wales.

The EU explained: “There would be a need for checks on goods travelling from the rest of the UK to Northern Ireland. There would be a need for some compliance checks with EU standards, consistent with risk, to protect consumers, economic traders and businesses in the Single Market.

“The EU and the UK have agreed to carry out these checks in the least intrusive way possible.”

So would this undermine the integrity of the United Kingdom, as some critics have claimed? That’s a matter of opinion.

Speaking to FactCheck, Michael Gordon, a professor of constitutional law at Liverpool University, pointed out that Northern Ireland is already a distinct part of the UK. It is already treated differently in certain ways.

“It seems to me quite well established that Northern Ireland has a specific, particular, distinctive constitutional status within the UK,” he said. “We have devolution, and Northern Ireland has devolved powers… That doesn’t mean it’s not part of the UK.”

However, some argue that the draft Brexit deal goes beyond the existing constitutional differences.

Professor of European Law at Cambridge University, Kenneth Armstrong, told FactCheck: “It’s not so much about the fact these are different rules – but who are the author of those rules.”

In other words, it’s one thing for domestic policies to create constitutional differences – but it’s another thing for them to be created by international agreements.

The ‘indefinite backstop’

Perhaps even more controversially is how long this backstop plan could last for.

Critics say that Mrs May’s deal does not give Britain the sovereign right to leave the backstop, without getting permission from the EU. This means the EU could keep the UK trapped in the backstop indefinitely, they say.

Conservative MP Jacob Rees-Mogg said: “It is also harder to leave this backstop than it is to leave the EU.”

Even Theresa May has admitted she shares some concerns about the backstop.

So what do the documents actually say?

Under the proposals, there is no mechanism for the UK to leave the backstop of its own accord. Instead, the UK and EU must “decide jointly” if they want to end it.

The draft deal says: “If… the Union and the United Kingdom decide jointly… that the Protocol, in whole or in part, is no longer necessary to achieve its objectives, the Protocol shall cease to apply.”

Professor Armstrong told us: “It doesn’t tell us what will happen if there’s disagreement.”

“On that basis, you might say it then becomes a semi-permanent mechanism… because there doesn’t appear to be a way out.”

So, critics are right to say the proposals don’t contain a specific, unilateral “get-out” clause. (This compares to membership of the EU, where Article 50 provides a specific mechanism to leave). But does this mean it would be completely impossible for the UK to leave the backstop?

The truth is, nobody can know for certain how difficult it would be to leave. But there are number of issues to bear in mind.

“The UK Parliament could, in strict UK constitutional terms, leave the Backstop by passing the appropriate Act of Parliament of its own accord,” explained Dr Joelle Grogan from Middlesex University’s School of Law (who contributed to the3million campaign, which campaigns for EU Citizens living in the UK).

However, she added: “This would only end the Backstop at a national level. At international level, the UK would be seen as in breach of its obligations under the Withdrawal Agreement.”

“Domestically, Parliament is sovereign,” Professor Gordon agreed. “Parliament can undo our commitment to that international agreement, in extreme circumstances. And it would be extreme circumstances because the costs of it would be so high…”

“The costs would be really, really significant – to the point where it probably should never happen. It’s not a safeguard, but it is a reality that I think qualifies any of these ideas that the EU could act in bad faith and lock us into some agreement at the international level. There would always be domestic law options.”

“The entire purpose of this mechanism is not to be permanent,” he added. “So when people say ‘the EU could trap us in this thing permanently by refusing to consent to an exit deal if we ask for one,’ that’s the point where I start to think this is actually approaching a misrepresentation a bit.”

“There are good faith negotiation obligations written into the terms of the withdrawal agreement.”

Perhaps more significantly, Professor Armstrong told FactCheck that the EU could find itself in legal difficulties if it trapped the UK in the backstop indefinitely.

The draft deal was negotiated under Article 50, which covers the withdrawal of member states. However, it is not meant to be used to establish a permanent future relationship. Therefore, if the backstop became permanent, that could be in conflict with the EU’s own laws.

“The European Union could not rely upon the permanency of that relationship through the backstop, because that itself would be a conflict with Article 50,” he said.

However, we could find ourselves in a Catch 22 situation. Professor Armstrong believes that, if the matter was taken to an arbitration panel, it would probably conclude the only legal means of ending the backstop would be by establishing a deal for the future relationship. But if no deal can be reached, we’d end up in deadlock.

What troubles Professor Armstrong more is that the proposals may have gone too far, in terms of planning for a future deal.

“The backstop is also being used, somewhat, as leverage for what the future relationship would look like,” he said. “We’ve got seven pages of bullet points on the future relationship that is yet to be negotiated, and it’s clear that there are very different views on that.”

He explained: “The backstop has ended up not just being a kind of insurance or safety net – but for some it’s a trap… for others it’s a way of kind of bouncing the UK into a particular version of the future relationship.”

There are clearly some very legitimate concerns about the constitutional implications of Theresa May’s draft Brexit deal, due to it not containing a specific mechanism for a unilateral end to the backstop. There is little doubt that – in theory – this could make life difficult for the UK, if the EU were to refuse to end the backstop.

But, as ever, the truth is a little more complicated than the headlines. If the UK did end up in this situation, there would be certain legal options we could explore – although it’s far from clear how this would pan out. Leaving without the EU’s consent would certainly be extremely difficult and almost certainly economically damaging. But it is not necessarily categorically impossible.

However things may pan out in the future, the draft proposals make clear that neither side currently intends to use the backstop indefinitely. Professor Armstrong told us: “I really do think this is not intended to be a trap but really a trampoline, and it will propel both the UK and the EU forwards to a new relationship.”