The activists were checked by security when they entered the building
Five climate change activists unfurled banners on Parliament's roof to protest against proposed Heathrow expansion, a court has heard.
Westminster magistrates were told that the Plane Stupid campaigners sneaked the banners past security in February.
Leo Murray, Graham Thompson, Alexander George, Tamsin Omond, all from London, and Olivia Chessell, from Cambridge, have denied trespass.
The protesters may have had "inside help", the prosecution said.
Except for Ms Omond, the other four are representing themselves in court.
Alex Lewis, prosecuting, said the activists unveiled two banners reading "BAA HQ" and "No Third Runway".
"All five were seen on CCTV going through St Stephen's that morning," she said.
But because all five were searched by security who did not find the banners, she suggested that the group had inside help.
In a statement, a security guard named as Miss McDonald, said she saw all five before the protest in the terrace cafe with "a tidy-looking chap" who appeared to be wearing a House of Lords pass.
Giving evidence, Insp Timothy Barfoot, who was on duty at the time, said one of the protestors contacted him after they had reached the roof.
"My recollection is he explained they were from Plane Stupid. He made the point that everyone on the roof had been trained in non-violent protest."
Mr Barfoot added that it was decided not to forcibly remove them over safety concerns as they were "close to the edge".
Replying to Mr Thompson, one of the accused, he said "apart from the litter of the paper aeroplanes" the building was not harmed.
Mr Murray told the court: "We knew our actions would be breaking Section 128 of the Policing Act."
But he said there was no other way to prevent the "greater crime" of building a third runway at Heathrow.
The trial continues.