Gatwick airport was at the centre of a major security alert last night after a live grenade was found in a passenger's luggage.

The grenade was discovered in the baggage of a 37-year-old Venezuelan man after he touched down on British Airways flight BA 2048 from Bogota in Colombia.

Passengers were immediately evacuated from the airport's north terminal and outbound flights were suspended. The terminal remained closed for several hours, affecting 100 flights.

The man, who was arrested for suspected terrorist offences, was moved to London where he was last night being questioned by Scotland Yard anti-terrorist police.

The latest incident came after the unrelated arrest under the Terrorism Act of two men near Heathrow during the continuing combined Army and police operation at the airport.

The day's dramatic events prompted Home Secretary David Blunkett to stress that the current terrorist threat was real and that the security services were "on the ball".

"It reinforces that we really do have a problem because that's the thing people have been saying - that we are making it all up, which is absolute nonsense," he said.

The Gatwick grenade was found as the man went through customs at the North Terminal.

BA said it was believed to have been in his luggage from the aircraft hold rather than his hand luggage.

When the grenade was found, explosives officers were called in and it was found to be live.

The plane, a Boeing 777 with 125 passengers on board, had stopped in Barbados. It is understood that the man arrested had boarded the flight in Venezuela.

The two men held near Heathrow were arrested when their car was stopped in a routine check in nearby Hounslow. Police described the arrests as "precautionary" and not linked to any plot to attack the airport.

At Stansted Airport, the access road is to be closed to the public from this morning due to increased security measures. Police and airport authorities said it was not because of a specific threat.

Conservative leader Iain Duncan Smith earlier accused the Government of fuelling fear among the public by sending out mixed messages about the level of the threat, but Mr Blunkett in turn accused the Opposition of "party political grandstanding".

Mr Blunkett, under pressure from the Tories, was forced to make a statement about the Heathrow alert in the Commons, where he said Britain faced a "real and serious" threat from al Qaida.

But he refused to reveal any more details, claiming it would not be responsible "to provide a running commentary" on the alert.

The Home Secretary defended his reluctance to come to the Commons saying: "Our view is that we must do nothing to undermine the work of the police and security services. We have to make fine judgements which must ensure the safety of sources of information.

"The terrorists must not be able to assess what we know and how we know it."