A parliamentary insider has been arrested following Wednesday's security breach at Westminster.
The 26-year-old man was arrested on Thursday evening on suspicion of aiding and abetting a criminal offence of trespass.
The five anti-Heathrow protesters who scaled the roof at the Palace of Westminster had claimed to have entered parliament as normal visitors and gained access to the roof via a fire escape.
However, the Metropolitan Police said there was a "strong possibility" they were the guests of a parliamentary pass holder.
The Met has now arrested a man who has worked with both MPs and peers.
He was questioned at a central London police station and bailed to return in a month.
The five protesters, all from campaign group Plane Stupid, have already been arrested under the Serious and Organised Crime and Police Act and will return to a central London police station for further questioning in April.
Wednesday's protest, the second in a week from anti-Heathrow expansion campaigners, marked the close of the government's consultation on the proposed third runway for the west London airport.
The protesters claimed the process was undemocratic as the government had already decided to go ahead with the runway, despite concerns this would adversely affect climate change, and "colluded" with airport operator BBA on drafting the consultation.
Gordon Brown, taking questions from MPs during the protest, said it was important decisions were taken on the floor of the House of Commons and not on the roof.