Brown denounced the protest during his weekly questions session in the Commons.
"The message should go out today very clearly that decisions in this country should be made in the chamber of this house and not on the roof of this house,'' he said.
George said the group gained access to Parliament as visitors, who must be invited by a legislator or accompanied by a security pass holder. They then walked through the building and went to a roof.
"We went up in a lift and walked out onto the roof. It was as simple as that,'' George said.
Brown's spokesman, Michael Ellam, said the government backs a third runway for Heathrow.
"The government supports the third runway at Heathrow in principle, provided strict environmental and noise conditions are met,'' Ellam said.
But he declined to comment on the apparent security breach.
"It's not clear what the full facts are at this point,'' he said. "Parliamentary security is a matter for the House authorities.''
The parliamentary department responsible for security said it could not immediately explain how the protesters were able to carry out the demonstration. Police said they were investigating the security breach.
Security in the Houses of Parliament -- home to both the House of Commons and the House of Lords -- has been tightened following a spate of intrusions in recent years.
Otis Ferry, son of rock star Bryan Ferry, and seven other men sneaked into the Commons chamber in 2004 in a pro-fox hunting protest. Months earlier, fathers' rights campaigners hurled purple powder at then-prime minister Tony Blair from the public gallery.
