At 11pm UK time on 31 October the UK would, by default, become a “third country” in terms of relations with the EU, with no post-Brexit plan in place, and no transition period. The UK would no longer be paying into the EU budget, nor would it hand over the £39bn divorce payment.

The UK would drop out of countless arrangements, pacts and treaties, covering everything from tariffs to the movement of people, foodstuffs, other goods and data, to numerous specific deals on things such as aviation, and policing and security. Without an overall withdrawal agreement each element would need to be agreed. In the immediate aftermath, without a deal the UK would trade with the EU on the default terms of the World Trade Organization (WTO), including tariffs on agricultural goods.

The UK government has already indicated that it will set low or no tariffs on goods coming into the country. This would lower the price of imports – making it harder for British manufacturers to compete with foreign goods. If the UK sets the tariffs to zero on goods coming in from the EU, under WTO ‘most favoured nation’ rules it must also offer the same zero tariffs to other countries.

WTO rules only cover goods – they do not apply to financial services, a significant part of the UK’s economy. Trading under WTO rules will also require border checks, which could cause delays at ports, and a severe challenge to the peace process in Ireland without alternative arrangements in place to avoid a hard border.

Some ‘no deal’ supporters have claimed that the UK can use article 24 of the General Agreement on Tariffs and Trade (Gatt) to force the EU to accept a period of up to ten years where there are no tariffs while a free trade agreement (FTA) is negotiated. However, the UK cannot invoke article 24 unilaterally – the EU would have to agree to it. In previous cases where the article has been used, the two sides had a deal in place, and it has never been used to replicate something of the scale and complexity of the EU and the UK’s trading relationship.

The Director General of the WTO, Roberto Azevêdo, has told Prospect magazine that “in simple factual terms in this scenario, you could expect to see the application of tariffs between the UK and EU where currently there are none.”

Until some agreements are in place, a ‘no deal’ scenario will place extra overheads on UK businesses – for example the current government advice is that all drivers, including lorries and commercial vehicles, will require extra documentation to be able to drive in Europe after 31 October if there is no deal. Those arguing for a ‘managed’ no deal envisage that a range of smaller sector-by-sector bilateral agreements could be quickly put into place as mutual self-interest between the UK and EU to avoid introducing or rapidly remove this kind of bureaucracy.

Martin Belam