What happens next if Theresa May's Brexit deal passes?

It has already caused two cabinet ministers to resign, sending shockwaves through Theresa May’s government. But if the Brexit deal passes – still a big if – the UK could face more political turmoil when it reaches another fork in the road in the summer of 2020.

By that time the UK would have left the European Union and be in the final months of a Brexit “pay-but-no-say” transition. By then it would be clear if the EU and UK are on the verge of a trade deal that the Brexit supporter Liam Fox claimed would be “the easiest in human history”.

If a trade deal is not ready, the UK would have to choose between two unpalatable options – extending the Brexit transition period, described as “purgatory” by leading Brexiters, or falling back on to an insurance plan that another Brexit supporter likened to Dante’s “first circle of hell”. The EU also sees this plan C as “suboptimal”.

And if there was no trade deal by the end of any extended transition, that insurance plan – the much-discussed Irish backstop – would fall into place.

What is the Brexit deal?

The Brexit deal is two documents. The first is a 585-page legal agreement to unwind the 45-year year relationship with Europe. The second is a seven-page outline describing the future relationship, which would be supplemented by a longer version to be agreed by the EU early next week.

For opponents of Brexit this confirms their worst fears of a “blind Brexit”. Sir Simon Fraser, a former permanent secretary at the Foreign Office, described the future relationship text as “utterly feeble” and a “laundry list” with no clarity on the long-term destination.

What have negotiators agreed on the Irish backstop?

The backstop is an insurance plan to avoid a hard border on the island of Ireland. If negotiators run out of time to find an answer through the future trading relationship, the EU and UK would enter a customs union, with extra demands on Northern Ireland to apply EU law on goods.

The backstop is like a Russian doll: a more elaborate legal arrangement for Northern Ireland has been tucked inside the larger all-UK provisions. The arrangement would apply “unless and until” it was suspended or superseded. It could not be broken unilaterally by the UK. While it would be temporary, it would have no expiry date and could only be ended by mutual agreement – a point that has riled Brexit and remain supporters.

The UK, locked into this customs union indefinitely, would have to apply the EU’s tariff and would be unable to sign free-trade deals on goods with other countries.

Is the backstop a ploy to keep the UK in by the backdoor?

The offer of an all-UK customs union is a reluctant concession to the UK. The EU has been wary of the backstop, which it has seen as special treatment for Northern Ireland, because businesses in the region would be allowed to access the EU market without restrictions.

The price of the backstop for the UK has been a series of guarantees that the government would not undercut the EU by becoming a tax haven or ditching worker rights or environmental protections. To that end, the UK has agreed to accept EU standards on state aid, competition, some taxation, environment and labour standards.

It is a promise that will keep the UK in the EU’s powerful regulatory orbit. The European commission, for example, would retain the power to overturn a UK government decision to subsidise a factory in Northern Ireland, if deemed to be illegal state aid.

What about extending the transition?

Negotiations on the UK’s future relationship with the EU would start on 30 March 2019 – the day after Brexit day. They would take place during a 21-month standstill transition period, when the UK would continue to follow all EU law while having no ministers, MEPs or any other decision-makers at the table.

Extending the transition is the one open question in the withdrawal treaty: the transition could be extended only once until “31 December 20XX”. That “XX” is to be agreed in the coming days.

It is a classic Goldilocks problem: too short and the transition would not be worth the negotiating effort; too long and both sides would be unhappy with the world’s sixth largest economy trapped in a broken-down marriage with the EU.

What about fisheries?

Negotiators have agreed to park the contested issue of fishing quotas, with the aim of reaching a deal before July 2020. EU member states want any deal on tariff-free access for British goods to be traded in exchange for maintaining existing fishing rights over the 100 common species that swim between the UK and rest of the EU.

The European commission managed to persuade member states to put this demand on ice until future trade talks, rather than link it to the Irish backstop. But France and the Netherlands remain unhappy about deferring the question, although other member states have been urging them not to compromise a Brexit deal over fish.

What happened to citizens rights and money?

Much of the Brexit accord was agreed months ago, with early wins to guarantee citizens’ rights and settle British debts and IOUs to the EU budget – the Brexit bill.

Citizen campaign groups on both sides of the channel have expressed bitter disappointment about the outcome. The3million, a group representing EU nationals in the UK, said existing rights were not fully protected. British in Europe, which has been campaigning on behalf of one million British citizens in the EU, said it was “upsetting” that rights to onward movement had not been included. This means a Briton living in Germany would no longer have the right to move to any other EU member state.

What else is in the smallprint?

• Gibraltar would be treated like the rest of the UK until the end of the transition period. Britain and Spain have agreed to work together to tackle smuggling and tax avoidance on the peninsula, which Spain claims as its own.

• The UK has agreed to honour protected status for Parma ham and champagne, among 3,000 other products. In turn, UK produce such as Mowbray pork pies and whisky would benefit from EU intellectual property rules. While the UK hoped to use this as a bargaining lever, the government has accepted the EU position.

What happens now?

Nobody knows whether the British parliament will support this deal. Some EU member states may also raise problems, although all players have agreed that the true hurdle lies in London, not EU capitals. A “better” deal was not on offer, EU officials have said.

Even if the Brexit deal becomes law, it would be only the end of the beginning of the road out of the EU.