LONDON (AFP/Reuters) - Britain downgraded its terror threat level to substantial on Monday, its lowest for at least three years, but stressed that it still faced a real and serious threat. The new level means an attack is a strong possibility, but is down from severe, meaning an attack is highly likely in Britain, four years after suicide bombers killed 56 people in London. I urge the public to remain vigilant, said Home Secretary Alan Johnson, adding: The police and security services are continuing in their thorough efforts to discover, track and disrupt terrorist activity. The five-grade threat level was first made public on August 1, 2006, when it was set at severe. British authorities define the international terrorist threat as stemming from diverse groups include al Qaeda, associated networks, and those who share its ideology but have no direct contact with it. We still face a real and serious threat from terrorists and the public will notice little difference in the security measures that are in place, Alan Johnson said. It was twice briefly raised to the top critical level meaning an attack is expected imminently first on August 10, 2006, after a series of arrests linked to an alleged plot to down transatlantic airliners; and again at the time of failed attacks in London and Glasgow in June 2007. But it has not been as low as substantial since the threat level was made public. Johnson said the decision to lower the level had been taken by the Joint Terrorism Analysis Centre (JTAC), a unit within the MI5 domestic intelligence agency. This means that an attack on the UK is a strong possibility. JTAC make their judgments based on a broad range of factors, including the intent and capabilities of international terrorist groups in the UK. We still face a real and serious threat from terrorists and the public will notice little difference in the security measures that are in place, he said. The new assessment covers potential attacks by Al-Qaeda and other terrorist groups, as well as dissident militants in Northern Ireland, clouded by a resurgence of unrest 10 years after the Good Friday Agreement ended three decades of the so-called Troubles. Britain, a key US ally in the so-called war on terror launched after the September 11, 2001 attacks, unveiled a new counter-terrorism strategy in March this year. At that time officials said Britain faced an increased threat of a chemical or even nuclear terrorist attack, and highlighted the risks from extremists in Pakistan and Somalia. In April anti-terror police arrested 12 mostly Pakistani men in raids across northwest England described by Prime Minister Gordon Brown as part of a probe into a major terrorist plot. The men were all subsquently released without charge. Browns predecessor Tony Blair, who joined then US president George W Bushs controversial invasion of Iraq in 2003, denied that this decision had made Britain more of a terrorist target. But Brown distanced himself from Bush after taking office in June 2007, and has progressively withdrawn troops from southern Iraq, while switching the focus to threats from Afghanistan and Pakistan. JTAC can set five threat levels: critical means an attack is expected imminently; severe indicates that an attack is highly likely, while substantial means an attack is a strong possibility. The lower levels are moderate, indicating that an attack is possible but not likely, and low, meaning that an attack is considered unlikely.