Best for Britain has launched a legal challenge trying to force Theresa May to concede a second vote on Brexit.
The group says existing legislation guarantees a new referendum if the government presses ahead with plans to keep Britain a part of EU bodies such as the European Medicines Agency.
Prime Minister Mrs May has suggested the government would seek ongoing membership of some bodies in her recent speech on Brexit.
But Best for Britain claim that such a move would amount to a transfer of powers to Brussels, which would trigger a new referendum under the 2011 European Union Act.
The group plans to emulate tactics used by anti-Brexit campaigner Gina Miller in a court battle to force ministers to give Parliament a vote before triggering the Article 50 process for Britain to leave the EU.
"This is not about Leave or Remain. It’s about making a democratic choice according to our constitution, given that many are worried about Brexit," said Best for Britain CEO Eloise Todd.
Government sources told the Mail on Sunday that the legal challenge was "nonsensical", adding that the current EU Withdrawal Bill in Parliament revoked the 2011 legislation.
