Anti-war protester Brian Haw left his post outside Parliament yesterday to face another courtroom battle over his right to continue his long-running vigil.

Along with a group of supporters, Mr Haw attended London's Law Courts to resist an appeal by the Government against an earlier legal ruling which allowed him to carry on demonstrating.

In July last year, Mr Haw won a High Court action against new laws threatening his round-the-clock demonstration, which he began in Parliament Square in June 2001. The court then ruled that legislation aimed at controlling demonstrations around the Houses of Parliament did not apply to 56-year-old Mr Haw.

But three senior judges in the Court of Appeal were urged by Home Secretary Charles Clarke yesterday to overturn that decision.

David Pannick QC, for Mr Clarke, told the Master of the Rolls, Sir Anthony Clarke, sitting with Lord Justice Laws and Lady Justice Hallett: "The appeal raises a short but important, and indeed interesting, question of statutory construction."

He said: "The question for the court is whether the provisions in the Serious Organised Crime and Police Act 2005, which now require authorisation for a demonstration in the designated area of Parliament Square, apply to a demonstration which began before the relevant provisions of the Act came into effect."

The High Court had upheld Mr Haw's submission, he added, that the answer "is in the negative" and it was that decision which was now being appealed against.

New rules state that anyone wanting to demonstrate in the area must have authorisation from the police "when the demonstration starts".

During the High Court action, lawyers for Mr Haw, who is from Worcestershire, pointed out that his demonstration had actually "started" four years previously and therefore did not have to apply for authorisation, even though the law was actually targeted at him.

Allowing his application for judicial review, Lady Justice Smith, sitting with Mr Justice McCombe and Mr Justice Simon, said the new law did not catch Mr Haw because of a drafting error.

She said: "If Parliament wishes to criminalise any particular activity, it must do so in clear terms. If it wishes to do so, Parliament can amend this Act."