A man with a rucksacks has run into the path of an aircraft at Heathrow Airport - sparking a major security alert.
He jumped the perimeter fence shortly after 2pm and was tackled by armed officers on the northern runway.
There was a controlled explosion on the bag and early reports are that no explosives were found.
"The northern runway is now being partially used while police deal with the incident," a BAA Heathrow spokesperson said.
"The southern runway is fully operational. Some delays are occurring as a result."
A British Airways spokesman said: "Flights will obviously be disrupted due to the closure of the runway."
Sky correspondent Mark White said the security scare comes ahead of the opening of a new terminal at Heathrow.
"Security is tight, especially with what is happening tomorrow morning with the opening of Terminal 5," he said.
"Her Majesty the Queen will be in attendance to open it and it will be a big day for BAA."
The state of the art new building is a major landmark in the airport's expansion.
However, a spokesman for climate change protest group Plane Stupid denied any involvement.
"What we know is that it was a lone man and he was carrying a rucksack, which is worrying in itself," White said.
"And he was determined to get airside."
He added: "The question the police will want to answer, and answer quickly, is what this man was doing airside."
Scotland Yard said anti-terrorist officers were aware of the incident but it was being dealt with by local police.
It is the second time in recent weeks security has been breached at the airport.
Protesters walked on to the runway last month and climbed on to a Boeing 777 before unfurling a banner in protest against the airport's expansion.
I thought I was cool
