If Theresa May’s Brexit deal is voted down, events will begin to unfold quickly

A no-deal scenario could result in months of chaos at British borders

If MPs decide to vote down Theresa May’s deal on Tuesday, events will begin to unfold quickly.

The prime minister will have just 21 days to seek further concessions from Brussels or put forward a different idea, giving her until New Year’s Day to come up with a plan B. In reality, she will need to put any new deal to MPs before parliament rises for Christmas on 20 December, or risk infuriating weary politicians by delaying the festive recess.

In theory, rejecting the deal ramps up the likelihood of a disorderly exit from the EU. A no-deal Brexit is the default scenario, so if no arrangement has been agreed by parliament by 29 March, then the UK will simply fall out of the EU without a deal.

A no-deal Brexit could result in months of chaos at British borders, with official warnings of the need to stockpile food and medicines.

In reality, only the most hardened Brexiteers want a no-deal Brexit, so many MPs are looking for ways to avoid such a scenario.

On 4 December, the Commons backed a move to allow them to amend whatever motion the government brings back, setting out their plans if Ms May’s deal is rejected.

The so-called Grieve amendment was laid by the former Tory attorney general – and unlikely rebel – Dominic Grieve, which gives MPs the chance to express a clear view of what should happen next.

This is significant because it could eliminate the prospect of a no deal, as there is no parliamentary majority for a chaotic exit.

MPs could choose to oppose no deal, support a new referendum or pursue a Norway-style exit that is advocated by growing numbers on Labour and Tory benches.

The Grieve amendment is not legally binding but it is effectively politically binding. Even in these extraordinary times, it would be almost impossible for the government to ignore such a vote.

None of these options command a parliamentary majority, however, so the way forward remains unclear.

Downing Street may be privately hoping the chaos from a defeat of the deal could clarify minds, especially if the pound slumps and businesses panic at the uncertainty.

Nervy MPs could be more likely to vote for a tweaked version of the Brexit deal, or some tougher wording in the political declaration in such circumstances – while some Tories could decide rebelling against the government once is enough to demonstrate their concerns.

Another scenario is an early election, as Labour has vowed to table a no-confidence motion in the government if the vote is lost.

But Jeremy Corbyn would need the support of Tory MPs to win this and they are unlikely to back him. Many still feel bruised by the 2017 election or fear they could lose another contest as Brexit chaos has prevented the prime minister from focusing attention on issues that matter to voters – such as social care, housing and schools.

A Final Say referendum is still a possibility but many MPs oppose it, and with a Tory leadership challenge still a possible outcome, all bets are off.