In order to prevent large legislative gaps appearing, EU law that affects the UK would need to be replicated or replaced with potentially divergent national measures. This process would be likely to involve enacting immediate transitional provisions, which the UK would have to bring in over a period of years, entailing inevitable issues and uncertainty.

As any directly effective treaty measure would cease to apply, new primary law would be needed to preserve any such measures.

The UK could create new laws so that the terms of directives and European Council regulations would have the status of Acts of Parliament, and European Commission (the Commission) regulations would have the status of statutory instruments. However, these EU laws typically assign ongoing roles to the Commission or another European body and it would be necessary to identify an appropriate minister or UK body to exercise that role in each case, or at least establish a methodology to do this.

In the case of UK measures that implement directives and supplement directly effective EU law, it would be necessary to take steps to preserve the UK's current position, whether in the form of primary legislation, statutory instruments under the European Communities Act 1972 (which would otherwise be repealed) or other UK statutes or the rules of a regulatory body, such as the Financial Conduct Authority (FCA). In other cases, provisions for co-operation with, or assistance to, the Commission might need to be removed.

Currently, these laws are governed by the EU approach to interpretation, which is purposive and has regard to recitals and headings. Parliament would need to decide if that approach should be applied to the co-opted laws, or whether they should be read according to UK statutory interpretation, which largely disregards those matters. Adopting the UK approach would increase uncertainty and speed up divergence from EU interpretation.

Linked to the question of interpretation, is the question whether courts in the UK should remain bound to follow Court of Justice of the European Union (CJEU) decisions, be bound to have regard to such decisions, or be free to depart from them. The UK courts will not (unless provided in a treaty with the EU) be able to seek decisions on interpretation from the CJEU where there is no clear answer. So a divergence of interpretation would be likely to increase in any event.

Financial services is a good example of a sector where the impact of a UK exit would be strongly felt. Almost the entirety of UK financial services legislation over the past ten years has EU legislation as its source. The move towards the European Single Rulebook has also meant that many EU rules are now directly applicable in the UK, but following an exit, those regulations would no longer have effect.

It seems unlikely, although not impossible, that a UK exit would trigger a mass overhaul or repeal of EU-based financial services legislation, especially in cases where implementation is recent and involved costly systems and operational changes; for example, the Markets in Financial Instruments Regulation (600/2014/EU) and the Markets in Financial Instruments Directive II (2014/65/EU) (together, MiFID II), which are due to come into force in January 2017.

Currently, a UK authorised firm has the right to carry on business in another EEA state, with or without a branch, provided that it meets the requirements of the single market directive under which the activities will be carried out. This passporting right allows UK authorised firms free access to EU markets. Following an exit, the UK risks losing this right, the impact of which will vary depending on the relevant sector legislation.

Depending on the nature of the exit, leaving the EU could mean either restricted EU market access for the UK, with third country status following an exit, or continued access to EU markets but without the ability to vote on financial services legislation, for example, as an EEA or European Free Trade Association member.

What third country status means in practice will need to be considered on a legislation-by-legislation basis. As an example, looking forward, MiFID II allows member states the option to require third country firms to establish a branch in that state before being permitted to provide services to retail clients and certain professional clients. Therefore, UK firms wishing to provide services to these clients across the EU may be required to set up branches in different member states. Access to EU eligible counterparties and certain professional clients will be possible under MiFID II without establishing a branch, but will be subject to the delay and uncertainty involved in seeking a Commission decision on equivalence.

To maintain full access to EU markets, some firms would need to set up subsidiaries in the EU. Subsidiaries will have the EU passport but would come with costly capital implications. For other firms, it could be necessary to move parts of their operations out of the UK into the EU. EEA firms that are currently passported into the UK, or which would like to gain access to the UK market after the UK's exit from the EU, would face similar uncertainties.

The future of the UK listing and prospectus regime would be largely dictated by whether the Treasury and the FCA left in place the existing measures in UK law and regulation which replicate the EU legislative measures that created the financial services single market.

As the Main Market of the London Stock Exchange is a leading international listing venue and trading platform, UK authorities may continue to require adherence to international standards equivalent to the relevant EU directives' requirements, in order to maintain the global reach of the Main Market brand.

It is possible that the UK will lose its ability to participate in mutual recognition between member states, as regards prospectuses and announcement requirements (either in whole or in part).

Where an issuer is based outside the EU (as a UK company or financial institution would be, following any exit), an EU home member state already has the power to approve a non-EU prospectus if it has been drawn up in accordance with international standards that are equivalent to the requirements under the Prospectus Directive (2003/71/EC). The Commission has the power to decide if a third country's law or practice is sufficient to satisfy the EU equivalence test, and could make this determination in respect of UK law and practice.

A key question is whether particular contracts could be terminated as a result of an exit, especially as the changed commercial landscape could prompt parties to look for ways to exit those contracts that are no longer required or profitable. Any right of termination would depend on the terms of the relevant contract, including any force majeure or material adverse change clause, and any right to terminate on notice.

Another issue is interpretation of pre-existing contracts. For example, how would an obligation to comply with a specific piece of EU legislation be interpreted after a UK exit? Similarly, how should a choice of English law as the governing law of a contract be interpreted if, at the time of contracting, EU law formed part of English law but, at the time of performance, it did not? These are essentially questions of contractual intention.

In most cases, presumably a choice of English law would be interpreted to mean English law as it stands from time to time, subject to any variations, including such variations as may arise from an EU exit. Therefore the validity and effectiveness of any contractual choice of law is very unlikely to be affected by an EU exit. This also follows from the continued operation within the EU of the Rome I Regulation (593/2008/EC), which in effect enforces any choice of law made by contracting parties, and although Rome I would cease to have effect in the UK, the pre-Rome I rules are very similar. However, it is possible that where some key provision of EU law is essential to the operation of a contract, so that without it, the contract is inoperable or incapable of being performed as originally anticipated, a different result may follow.

We may see some companies seeking to include a specific provision in new contracts dealing with the consequences of a UK exit from the EU. There is also the question in relation to new contracts, as to whether any EU exit may have an effect on the enthusiasm of parties to choose English law as their governing law. This seems unlikely to be the case in many instances, given that English domestic commercial law has its own well-developed and respected rules, which have largely been unaffected by EU intervention.