David Cameron may be on the brink of brokering a new deal for the UK within the European Union. It could come this week, at the EU leaders’ summit on 18-19 February. If so, it is widely expected that the UK referendum on the nation's EU membership would be held on 23 June.

The question for UK voters will be this: 'Should the UK remain a member of the EU or leave it?' It’s a simple binary choice. But the reality is more complicated. If UK citizens vote to leave, big questions will still need to be answered. From a business perspective, among the most pressing are what the UK’s relationship with the EU would look like post-Brexit, and how the UK’s relationships with other countries would be affected.

Referendum voters won't be asked about these issues. They cannot even be told how they will be dealt with, as UK negotiations with the EU and the other countries concerned will only start after a vote to leave. And of course, the UK cannot simply choose its preferred scenario – the other countries also need to agree.

Weighing up the options

The various special relationships already in place between the EU and other countries illustrate the main possibilities for the UK post-Brexit – the Norwegian option, Swiss option and the Turkish option, among others. All of these would reduce the UK’s ability to access the EU market to some extent. The impact would range from relatively small in the case of the Norwegian option (the closest form of association), to possibly very significant in the case of the World Trade Organisation option (that is, no special agreement, and reliance simply on the UK’s rights and obligations as a WTO member) at the other end of the spectrum.

The more EU market access the UK were to retain, the more it would be obliged to accept market rules made by the EU. There is no free lunch. For example, under the Norwegian and Swiss options the 'four freedoms' of the EU single market would still apply, including free movement of workers, and the UK would still have to contribute to the EU budget. And even under the Norwegian model, the UK’s power to influence the formulation of those rules would be greatly reduced.

Parameters of the possible

The post-Brexit UK/EU relationship would not necessarily have to replicate one of the existing arrangements. However, the rules of the WTO set some parameters. The 'most favoured nation' concept generally prohibits members from agreeing to special trade terms with other countries that are not also offered to all other WTO members. Any post-Brexit trade agreements between the UK and the EU would need to fit within the exceptions. For example, a free trade agreement relating to goods would need to eliminate substantially all tariffs and other barriers to trade, and an agreement relating to services would need to have 'substantial' sectoral coverage. The UK and EU couldn't just put in place whatever arrangements they wanted.

Going it alone

There is also the question of the UK’s trade relationships with the rest of the world. Free trade agreements are negotiated by the EU as a trading bloc (currently there are about 60). The UK has the benefit and burden of them as an EU member. Outside the EU, the UK would most likely cease to be party to these and would have to negotiate its own agreements. Some argue that the UK would eventually be able to secure better deals.

Besides these external relationships, the UK would need to decide what to do about the large body of UK law that derives from EU legislation. If the UK remained a member of the European Economic Area (as under the Norwegian option) it would have to retain a lot of this. But in other cases the UK would have greater flexibility to fashion its own laws as it saw fit. A huge and complex review exercise would have to be undertaken. Even establishing legal certainty for an interim period pending completion of the review would be a highly complex business.

Looking forward

In the face of these big questions, lawyers can help clients to begin to reduce uncertainties, to quantify and assess what different forms of Brexit might mean for them and to help them think through their responses. Some legal changes would potentially impact a wide range of businesses (for example in relation to choice of law, jurisdiction, employment and data protection), although of course they will affect different firms in different ways. There are also particular industries that are deeply shaped by sector-specific EU regulatory regimes—for example financial services, media, telecoms and energy. There is no real substitute for an in-depth analysis, issue by issue and business by business, to get a full view.

Andrew Marsh is a financial services lawyer based in Freshfields Bruckhaus Deringer's London office and a member of the firm’s Britain and the EU team.