The law used to push through a ban on hunting with dogs is "invalid" because "it is not truly an Act of Parliament", the High Court has been told.
The ban on hunting is due to be implemented in February
The fundamental legal flaw will mean thousands of people losing livelihoods and "a major source of recreation and enjoyment", the court heard on Tuesday.
Sir Sydney Kentridge QC made the claim for the Countryside Alliance at the start of a legal challenge to the ban.
The ban, affecting England and Wales, is due to come into force in February.
But the QC said its validity depended on the validity of the 1949 Parliament Act, which was used by the House of Commons to overrule objections by the House of Lords.
Sir Sydney told Lord Justice Maurice Kay and Mr Justice Collins: "We submit that it is invalid and that, despite its title, it is not an Act of Parliament."
The application for judicial review is in the names of John Jackson, Countryside Alliance chairman, and Mair Hughes, from Gilfach Goch, Mid Glamorgan, wife of the Master of the LLangeinor Hunt, who is also a farrier.
Mrs Hughes, 46, claims her job as book-keeper to the farriery is in danger from the ban, due to come into effect on 18 February, as well as her social life.
Sir Sydney told the hearing that the importance of the issue was evident from the speed with which the case had come to court and the fact the Attorney General, Lord Goldsmith QC, was appearing in person to defend the legislation.
He said if the Hunting Act, which introduces the ban, becomes law, "it will affect the livelihood of numbers of persons present, including two of the applicants".
"It will prevent many thousands more from contiuing lawfully what has been for many a major source of recreation and enjoyment," he said.
Outside the court, Mr Jackson said the Commons had no right to pass laws banning hunting and described the 1949 Parliament Act as "unlawful".
However, the government has said it is "confident" that the courts will decide to uphold the ban.
If this week's initial challenge fails, the Countryside Alliance has said it will seek an injunction to allow further appeals to take place.
But Mr Jackson said: "We're not expecting to lose."
In a press conference, he said: "This is probably the most important constitutional case the High Court has ever had to consider and it's something which should concern all of us."
Anti-hunt protesters, the League Against Cruel Sports are also in court to make legal representations against any injunction.
Rural Affairs Minister Alun Michael has previously told MPs he did not believe the challenge to the ban would succeed.
The government, however, has said it will not oppose an injunction while the court process continues - a move which led to accusations it was trying to avoid protests so close to a likely May election.
Conservative frontbencher James Gray condemned as a "grubby political ploy" Downing Street's decision to tell reporters it would not oppose an injunction.
The League Against Cruel Sports chairman John Cooper said they were going to court "seeking to uphold parliamentary democracy by opposing any delay to the commencement of the ban lasting beyond February 18, the date agreed by Parliament".