How reports of a planned 'spectacular' triggered a huge security operation

This week's security operation – one of the most extensive in recent British history – began after GCHQ, the intelligence service's listening post, began to decipher reports of an imminent "spectacular" by Islamist terrorists in London.

Almost simultaneously MI5 is believed to have discovered that militants connected to al-Qa'ida have been in touch with sympathisers in Britain plotting to smuggle shoulder-launched Sam-7 surface-to-air missiles into the country.

The information was passed by MI5 to Scotland Yard's Special Branch, and thence to the Commissioner of the Metropolitan Police and the Home Office.

The Cabinet Office's civil contingency committee, Cobra ( Cabinet Office Briefing Room One), was convened to consider measures including the shutting down of Heathrow airport – but not at that stage Gatwick and Stansted – the deployment of armed police and troops in central London, and activating flights by Tornado GR-4 strike aircraft from RAF Marham over the capital.

The decision was ultimately taken to deploy 1,500 armed police and troops to Heathrow and fly one Nimrod MR2 reconnaissance aircraft on patrol.

Downing Street insisted that although it has sanctioned the use of troops, the decision on the mechanics of the deployment had been left to the police and the Ministry of Defence.

The decision to use more than 450 troops – from the 1st Battalion, the Grenadier Guards and the Household Cavalry – as well as light armour at Heathrow caused some raised eyebrows at the Ministry of Defence, although others insisted it was meant to be a deterrent.

The armour had been withdrawn by yesterday, but high-profile foot patrols continued at Heathrow and other airports, while smaller units conducted searches in surrounding areas.

The operation, which is expected to continue for several more days, is being co-ordinated by Scotland Yard's anti-terrorist branch and Special Branch, the Ministry of Defence, the Security Service (MI5) and the Secret Intelligence Service (MI6).

The available manpower – police and troops – can be increased or decreased at extremely short notice, and a number of army units, throughout the country, have been placed on alert.

Military eye in the sky

The Nimrod is, in terms of military technology, rather long in the tooth. It has, however, proved to be remarkably effective in a variety of conflicts from Kosovo to Afghanistan.

Its main role is maritime reconnaissance, including anti-submarine operations, but it can be used to monitor cars and people.

One of the main advantages of the Nimrod is that it can fly for up to 10 hours at a stretch, and the timescale can be extended through air-to-air refuelling.

The Nimrod's main role in the current anti-terrorist operations would be one of surveillance for both aircraft and movement on the ground. The advanced photographic and audio equipment on board can be used on suspicious vehicles and aircraft as well as people. The communications system will relay the information to Ministry of Defence sites to be passed on to the police and intelligence agencies.

The Nimrod is in the process of being updated with new engines and avionics, but engineers discovered the wings are too weak, delaying test flights from the end of last year to mid-2003.