UPDATE 5-Man sprints onto Heathrow runway in security breach

(Adds police believe incident not terrorism-related)

By Peter Graff

LONDON, March 13 (Reuters) - A man sprinted onto a runway at London's Heathrow airport on Thursday, the day before Queen Elizabeth was due to open its giant new Terminal Five, the second major security breach in less than a month.

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. (1405 GMT) 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.