A British think tank said yesterday it planned to take legal action over whether the government could take Britain out of the single market as part of its exit from the European Union, in the latest legal attempt to challenge the Brexit process.

The pro-single market group British Influence said it believed the government could be breaking the law if it did not get a clear legal opinion as to whether Britain’s membership of the EEA automatically ended along with its membership of the EU.

Membership of the EEA, which includes the EU countries as well as Norway, Iceland and Liechtenstein, provides access to the single market and its free movement of goods, capital, services and people.

“The single market was not on the ballot paper,” Jonathan Lis, the deputy director of British Influence, told Reuters. “We’re leaving the EU, that’s fine, but we don’t need to leave the single market and the government should be embracing this intervention – not disputing it.”

A spokeswoman for the government said Britain was only party to the EEA Agreement in its capacity as an EU member state. “Once we leave the European Union we will automatically cease to be a member of the EEA,” the spokeswoman said.

“The referendum result will be respected and we intend to invoke article 50 (the formal mechanism to leave the bloc) no later than the end of March next year.”

Theresa May’s government is already challenging a ruling by the High Court that parliamentary approval is required before article 50 can be triggered. The government has appealed the ruling and will make its case at the UK Supreme Court in early December.

Lis said British Influence was beginning to consult lawyers over its case and could not provide a likely timetable for any challenge. London’s High Court would also have to grant permission for any judicial review to be heard once the case was submitted for consideration.