Timeline: UK terrorism threat levels explained

The UK government has raised the threat level of terrorism to severe - one short of its highest possible level. The level moves from time to time based on the recommendations of the Joint Terrorism Analysis Centre (JTAC).

There are five levels of threat and "severe" means a future terrorist attack is "highly likely", although not necessarily imminent.

The threat level does not mean that the public should necessarily change their behaviour, other than remain vigilant. But it does help police forces and other agencies to plan the security measures they may need to put in place.

Pre 7 JULY 2005: SUBSTANTIAL

The old system for explaining the threat level was more complicated than the current one. Weeks before the 7 July attacks on London, the level was lowered from what was then called "severe general" to "substantial". One of the decisions that came in the wake of the attacks was to simplify the threat level and make it public.

1 AUGUST 2006: SEVERE

The threat level was officially published for the first time just over a year after the attacks on London. Its five levels closely mirrored the US system. The then home secretary John Reid told Parliament at the time that the system was not an exact science but hoped it would help the public understand the threats.

10 AUGUST 2006: RAISED TO CRITICAL

The level was raised as the security services smashed the massive plot to bring down transatlantic airliners with homemade liquid bombs. While the police had successfully rounded up the three ringleaders, convicted last autumn, it wasn't clear at the time whether there were other cells operating elsewhere to the same plan. The raising of the level came as sweeping new security measures were introduced at British airports - restrictions that remain in place today.

14 August 2006: LOWERED TO SEVERE

As the security situation calmed down in the wake of the airlines bomb plot arrest, the Joint Terrorism Analysis Centre recommended lowering the level one notch.

"There has now been time to assess the intelligence picture following the police operation," said John Reid at the time. "The police believe that the main suspects in the alleged plot were arrested last week."

30 JUNE 2007: RAISED TO CRITICAL

This change in status came after London witnessed a botched attempt to bomb a nightclub, followed by the failed attempt to ram a car into Glasgow Airport. One of the bombers, Kafeel Ahmed, died of his injuries. His accomplice, Bilal Abdulla, is now serving a life sentence.

4 JULY 2007: LOWERED TO SEVERE

The level of threat came back down after a fresh analysis of the situation by the Joint Terrorism Assessment Centre. The investigation into the London and Glasgow attacks was progressing rapidly - but it was becoming apparent who had been behind the plot.

20 JULY 2009: LOWERED TO SUBSTANTIAL

No specific information is in the public domain about the reasons for the lowering of the status during the summer of 2009 - although it was clear that there had not been any major counter-terrorism arrests by police for some months.

The Home Secretary, Alan Johnson, stressed that it was essential the public did not drop their guard. "The change in the threat level to substantial does not mean the overall threat has gone away," he said. "There remains a real and serious threat against the United Kingdom and I would ask that the public remain vigilant."

22 JANUARY 2010: RAISED TO SEVERE

Alan Johnson announces the level has been raised to severe in the wake of the foiled attempt to blow up an airliner heading for Detroit.