Army guard at Heathrow?

Airport bosses are facing questions today over how a man was able to scale the perimeter fence and come within yards of a packed passenger jet.

An inquiry into the security breach was launched after the 26-year-old Sri Lankan was arrested at the airport yesterday. The intruder, who is believed to be an asylum seeker, was surrounded by armed police after he sprinted into the path of an Emirates jet as it took off on the northern runway.

The officers were alerted by a radio message from the Emirates pilot as he watched the man running towards him.

The intruder was carrying a rucksack, triggering fears that he was a suicide bomber.

The jet took off seconds before the man reached it, but others were halted on the Tarmac as he was arrested. Police carried out a controlled explosion on the bag the man was carrying but it was found to contain only clothes.

Sources said the incident was not linked to terrorism and the man was not a protester linked to opposition to Terminal 5.

The man, who speaks Tamil but no English, was detained for being airside illegally. His mental health was being assessed by doctors today.

The ease with which the man evaded detection and was able to scale the 15ft fence and reach the northern runway is deeply worrying for police and security officials. While police officers patrol the airport, security is the responsibility of BAA, Heathrow's owner.

It was the second breach at the airport within 18 days and experts said it would lead to calls for the military to patrol the perimeters. One security insider said: "The matter is worrying. What if he had been a suicide bomber and had reached the plane? The consequences could have been catastrophic."

The breach was particularly embarrassingfor BAA as it took place while security procedures were being tested in preparation for the Queen's visit. Officials carried out an urgent review of the plans for the visit last night.

Senior officers at Scotland Yard have been calling for improved security at the airport for years. In 2006, a chief superintendent at Heathrow said security had not kept pace with the terrorist threat. Jerry Saville said he was concerned about the effectiveness of perimeter fences.

A BAA spokesman said: "Clearly this incident is a matter of some concern." He added that it was co-operating fully with an investigation.

Yesterday's incident, at 2.05pm, came less than three weeks after Greenpeace protesters climbed ontop of a passenger jet. Two days later members of the Plane Stupid group breached security at Westminster, climbing onto the roof and unfurling banners. Both groups denied involvement in yesterday's breach.

Buckingham Palace said today's visit by the Queen, accompanied by the Duke of Edinburgh, would go ahead.