Colouring books were among the items police removed in searches
Kent Police have admitted they broke the law when they stopped and searched some people at Kingsnorth Climate Camp.
Judges at London's High Court were shown a confidential police document that was given to officers dealing with the protest.
Lawyers for the force told the court they accepted the note did not provide a lawful basis for searches.
Kent Police have agreed to settle a claim for damages brought by two twins and an environmental campaigner.
The note, which outlined the force's stop and search policy for officers on the ground, stated: "Intelligence and information received directly from protesters' websites in the lead-up to Climate Camp, that they intend to break the law to attack the power station, gives the police reasonable grounds.
"Further grounds may exist as a result of articles already seized."
Lord Justice Aikens and Mr Justice Openshaw said the police now accepted this was not a lawful basis for stopping people and it would be added into the statement of reasons for the settlement of the three cases.
The twins, referred to as E and T, were searched while at a demonstration against Kingsnorth coal-fired power station in August 2008.
Long-standing environmental campaigner David Morris, from north London, said he was forcibly searched against his will.
All three were among some 1,000 protesters at the camp near Hoo in August 2008.
John Halford, the solicitor acting for the twins and Mr Morris, said: "The note is a smoking gun. The police won't accept everyone else was searched unlawfully - but that is the logical conclusion of this new evidence."
The mother of the twins said: "I'm really pleased and I'm sure they'll be delighted when they get in from school and find out that they've been victorious.
"It [the search] should never have happened."
To perform a search under Section 1 of the Police and Criminal Evidence Act 1984 police must have a "reasonable suspicion" that an individual is carrying prohibited weapons or articles that could be used for criminal damage.
Police have admitted they did not have reasonable suspicion in the case of the twins and Mr Morris, but have not made any similar admissions about other people who were stopped and searched.