A fresh legal challenge over Brexit is being heard at the High Court today.
It is being brought by a group of campaigners who want a "soft Brexit" and to keep Britain in the European single market.
They include Peter Wilding, chairman of the pro-Europe pressure group British Influence, who is credited with coining the word Brexit.
He is joined by Conservative lobbyist Adrian Yalland, who voted Leave, and four anonymous applicants referred to as W, L, T and B.
In a historic ruling, the Supreme Court declared that Parliament must give its approval to trigger Brexit under Article 50 of the Lisbon Treaty.
The new case centres on Article 127 of the "separate treaty" relating to the UK's membership of the European Economic Area (EEA).
The EEA Agreement provides for the free movement of persons, goods, services and capital within the single market.
The applicants are asking the High Court in London for permission to seek a ruling that Parliament must separately give permission before the Government can pull out of the EEA.
They want a declaration that it would be unlawful for Prime Minister Theresa May to cause the UK to leave the EEA without prior Parliamentary authorisation through an Act of Parliament, and the serving of a withdrawal notice under Article 127.
The Department for Exiting the European Union says the case is unarguable and contends: "Once the UK leaves the EU, the EEA Agreement will automatically cease to apply to the UK."
A single judge who considered the case in private last December refused permission to apply for judicial review, but the applicants are exercising their right to renew their application in open court in a hearing before Lord Justice Lloyd Jones and Mr Justice Lewis.
Mr Yalland, whose partner is Polish, said: "I voted to leave the EU but Parliament did not intend the referendum to cover the issue of membership of the EEA."
He warned: "Hard Brexit risks Scotland leaving the Union. Brexit gives us our country back but a hard Brexit risks breaking it up."
Mr Yalland said he had "skimmed" the Government's white paper setting out its blueprint for leaving the EU, adding that "it offers no hope of a Parliamentary vote on the decision to leave the EEA".
He said: "We fundamentally disagree with the Government's legal position in law, and as a negotiating tactic.
"In conceding membership of the single market, the Government is throwing away its 'ace card' in the negotiations, as well as a viable transition measure.
"If we decide when we leave the single market, and it is at our discretion whether we do so or not, then the advantage in the Brexit negotiations is with us."