LONDON (Reuters) - A man sprinted onto a runway at Heathrow airport on Thursday, the day before the Queen was due to open its giant new Terminal Five, the second major security breach in less than a month.

View of Heathrow airport in London in this file picture. REUTERS/Luke MacGregor

Police arrested the man and blew up his bag in a controlled explosion but found no bomb. A spokeswoman said police did not believe the incident was terrorism-related, although they were still not certain of the man’s motive.

The world’s busiest international airport has been the target of protests from environmentalists angered at both the opening of the new terminal and further plans to boost traffic with the addition of a third runway and a sixth terminal.

“I can confirm that at approximately 2:05 p.m. a man was seen airside at Heathrow airport on the northern runway. He’s been arrested by police and he’s currently in custody,” a police spokesman said.

Last month protesters opposing the expansion plans breached the airport’s security to unveil a banner on top of a parked passenger jet. Others chained themselves to the roof of parliament. Organisers have threatened more action.

Television pictures showed several emergency cars with flashing lights parked on the airport runway.

About two hours after the incident airport operator BAA said both runways had reopened.

“Some short delays have occurred but we are expecting to return to normal later this afternoon,” said a spokeswoman for BAA, which is owned by Spanish firm Ferrovial.

A spokesman for the Queen said there were no plans to cancel her appearance at Friday’s opening ceremony for the new Terminal Five.

“At this stage there’s no reason to expect that it will not go ahead as planned,” a spokesman for the queen said.

Security at the airport has also been a high-profile issue since 2006 when authorities said they had thwarted a plot to smuggle explosives onto trans-Atlantic flights.

“The important thing about the Heathrow incident is that the person was detained, that all the security precautions went quickly into action, that he’s now being interviewed and that all possible steps were taken,” Prime Minister Gordon Brown told reporters at a European summit in Brussels.