The following cases provide an illustration of the nature of the current threat to the UK from Islamist extremists.
The fertiliser bomb plotters
In March 2004, the 'fertiliser bomb' plot was disrupted. The leader of the group, Omar Khyam, had planned to attack targets such as the Bluewater shopping centre in Kent or the Ministry of Sound nightclub in London with homemade explosives. Khyam had strong links with Al Qaida. Dhiren Barot, another Al Qaida-linked terrorist, was the ringleader of a separate cell disrupted in August 2004. His group planned to detonate "dirty bombs" containing radioactive material, which they were never actually in a position to obtain.
In 2005, the 7/7 bombings on the London transport system killed 52 people. The leader of the 7/7 group, Mohammad Sidique Khan, was a close associate of Omar Khyam and had undergone terrorist training in the Federally Administered Tribal Areas (FATA) of Pakistan. Muktar Said Ibrahim, the leader of the subsequent 21/7 failed attacks, also received training in Pakistan. The individuals in these plots were all supported and directed by Al Qaida in the FATA, and more specifically by British national Rashid Rauf who was head of Al Qaida's external operations at the time.
Alongside the 7/7 bombings, in 2005 a number of individuals were plotting to use liquid bombs to attack transatlantic passenger aircraft travelling between the UK and the US and Canada. Planning had reached an advanced stage when the plotters were arrested in August 2006. The leader of the group, Ahmed Ali Khan, had been directed by members of Al Qaida including Rashid Rauf and the plot's suspected mastermind, Abu Ubaida al-Masri.
In December 2010, nine men from London, Cardiff and Stoke were arrested for planning attacks against targets including the London Stock Exchange. In September the following year eleven men from Birmingham were arrested for plotting to conduct a bombing campaign using explosives in rucksacks and attached to timing devices.
In May 2013, Michael Adebolajo and Michael Adebowale attacked and killed off-duty soldier Fusilier Drummer Lee Rigby near the Royal Artillery Barracks in Woolwich. The men ran over Drummer Rigby with their car before murdering him. Adebolajo was known to have a history of involvement with Islamist extremism and was arrested in Kenya in 2010 under suspicion of trying to reach an al-Shabaab terrorist training camp.
Aspiring to imitate this murder, teenager Brusthom Ziamani was on his way to kill and behead a British soldier when he was arrested in August 2014. This was not the only Woolwich-inspired attack: a group of three men including Nadir Syed were planning a Woolwich-style attack against military personnel at a remembrance day parade in November 2014. All three were arrested during the planning stages of their attack.
International terrorism is a nationwide problem in the UK. Attacks have occurred in London and Glasgow, and thwarted terrorist plots have been aimed at targets outside the capital.
For notable terrorist convictions since the beginning of 2002, see our News section.
Counter-terrorism statistics relating to arrests and their outcomes are published by the Home Office.