What happens if there's no trade deal by 31 December?

When transition ends on 31 December, the UK will automatically drop out of the EU's main trading arrangements (the single market and the customs union).

The single market means that countries share the same rules on product standards and access to services, whereas the customs union is an agreement between EU countries not to charge taxes (tariffs) on each other's goods.

However, if a new UK-EU trade deal is not agreed in time then tariffs and border checks would be applied to UK goods travelling to the EU - under the rules of the World Trade Organization. The UK also decides what tariffs and checks to impose on EU goods.

Tariffs would make UK goods more expensive and harder to sell in the EU, while full border checks could cause long delays at ports.

Failure to reach a deal would also result in the UK service industry losing its guaranteed access to the EU. This would affect everyone from bankers and lawyers, to musicians and chefs.

Even if a trade deal is reached, it would not eliminate all checks - so UK businesses will need to prepare.

As well as trade, other aspects of the future relationship - including fishing access and security co-operation - also need to be signed off. If not, then no-deal plans will be needed in these areas for 1 January.

Any trade deals the UK strikes with non-EU countries will be allowed to start from 1 January (ie once the UK has left the EU customs union). Brexit supporters have long argued that allowing the UK freedom to set its own trade policy will benefit the economy - although critics say it's more important to remain close to the EU.

(source https://www.bbc.co.uk/news/uk-politics-32810887)