The security operation at Heathrow, which saw several hundred troops patrolling the airport for several days, has been scaled down.

However, security sources say the threat to Britain remains high.

None of those suspected of plotting to shoot down an aircraft has yet been caught - and a missile launcher possibly smuggled into the UK has not been found.

About 450 troops, equipped with light tanks, were deployed at Heathrow last week along with about 1,000 police, after intelligence reports suggested terrorists may be plotting a missile attack on a passenger plane.

Somebody may have left the country; some piece of intelligence has just redirected and refocused the operation Roy Ramm, former head of Scotland Yard

The operation coincided with the Muslim festival of Eid, which ended on Saturday.

The troops have now been put on standby, although a high police presence remains at the airport.

Security expert Roy Ramm - a former commander of the Metropolitan Police - said this must mean intelligence reports had changed.

"The threat last week to Heathrow clearly included a threat from a missile.

"This week, something from intelligence has driven that kind of operation under cover again.

More alerts likely

"We don't know for sure what police and the security services are doing, but somebody may have left the country; some piece of intelligence has just redirected and refocused the operation."

Although Britain's intelligence agencies are working together to counter the threat, more such alerts are likely in future, said BBC security correspondent Frank Gardner.

"With al-Qaeda bent on a long-term campaign against the west, we can expect similar scares in the months to come. The so-called war on terror has no foreseeable end."

During the alert, Gatwick Airport was evacuated and many flights cancelled after a passenger allegedly tried to smuggle a live grenade through customs.

Hasil Mohammed Rahaham-Alan, 37, from Venezuela, was on Monday remanded in custody charged with three terrorism offences.