Under EU rules, EU member states cannot make separate trade deals with individual member states or non-EU countries on their own.
However, there is no legal precedent for a country to leave the EU and therefore need to renegotiate any trade agreement.
It is unclear whether invoking Article 50 would result in a change to the UK’s membership status, and therefore permit trade negotiations to begin with the EU and other non-EU countries (including those who already have agreements with the EU).
Brexit Mythbuster 