The rights of those involved in public demonstrations have been clarified in a House of Lords ruling in which the Lord Chancellor, Lord Irvine, laid down important guidelines. It is only his second judgment in the Lords since taking office.

The case of the Director of Public Prosecutions v Jones & anor is of major significance and the Lords have given the highest-level guidance so far on the interpretation of the "trespassory assembly" provisions of the 1986 Public Order Act (later incorporated into the 1994 Criminal Justice and Public Order Act).

Angus McBride of Bristol-based Douglas & Partners, a specialist in environmental protest cases who represented one of the appellants in the case, says the decision will make it very difficult for the police to enforce the provisions of the Act if it can be shown that an assembly behaved peaceably and was non-obstructive.

The original provisions date back to the 1985 "Battle of the Bean Field", in which arrests were made when a group of travellers was broken up as they converged on Stonehenge for the summer solstice. In a bid to arm the authorities with powers to counter future gatherings, provisions were included in the 1986 Act prohibiting "trespassory assembly" and defining the term as applying to an assembly that exceeded the public's rights of access.

It was these provisions that were brought into play in the summer of 1995 in a bid to curb gatherings of more than 20 people at Stonehenge. An order was made under the Act prohibiting gatherings at the site, but on the evening of 1 June that year, police counted 21 people in a group on the grass verge adjacent to the perimeter fence surrounding the ancient stones.

The protesters were told that they were in breach of the order banning gatherings of more than 20 people. When some refused to move, they were arrested.

Local magistrates later convicted them of the offence of trespassory assembly but the convictions were overturned on appeal to the Crown Court. The Crown Court judge took the view that since those arrested had not been guilty of destructive, violent or disorderly behaviour and had not done anything threatening a breach of the peace, their presence amounted to "reasonable use of the highway".

However, this decision was reversed by the High Court, which took the view that the public's right of access to the highway was to "pass and repass and to do anything incidental or ancillary to that right" and that an assembly of the type that took place, even though it was peaceful, could not be regarded as incidental or ancillary.

The Law Lords have now overruled the High Court's decision, by a three-to-two majority, after a challenge brought by two of those arrested, Dr Margaret Jones and student Richard Lloyd.

In his ruling, Lord Irvine said: "The public highway is a public place which the public may enjoy for any reasonable purpose, provided the activity in question does not amount to a public or private nuisance and does not obstruct the highway by unreasonably impeding the primary right of the public to pass and repass. Within these qualifications there is a public right of peaceful assembly on the highway."

Aggrieved landowners, however, may take comfort from the comments of Lord Hutton's judgment in the case. Lord Hutton, although he agreed with Lord Irvine, said he did not consider that "peaceful and non-obstructive public assembly" could always be classed as reasonable use of the highway and each case should be decided individually.

"If members of the public took part in an assembly on a highway but the highway was, for example, a small, quiet country road or was a bridle way or a footpath, and the assembly interfered with the landowner's enjoyment of the land across which the highway ran or which it bordered," he said, "I think it would be open to the justices to hold that, notwithstanding the importance of the democratic right to hold a public assembly, nevertheless in the particular circumstances of the case the assembly was an unreasonable use of the highway and therefore constituted trespass."

Despite these comments, McBride says he believes that the ruling has clarified the situation. "The decision will make it very difficult for police to enforce orders under the Act," he says. "However many people are gathered, if they are gathered in a peaceful and non-obstructive way, then I do not believe they can be said, in the light of this judgment, to be a trespassory assembly."