Lawyers say uncertainty over the UK's European Economic Area membership means ministers could be stopped from taking Britain out of the single market.
They will argue the UK will not leave the EEA automatically when it leaves the EU and Parliament should decide.
But the government said EEA membership ends when the UK leaves the EU.
The single market allows the tariff-free movement of goods, services, money and people within the European Union, as if it was a single country.
