British police appealed on Monday for tip-offs from the public on any illegal firearms in circulation, stepping up efforts to prevent what they described as a "marauding terrorist attack" similar to the Paris attacks a year ago.



London and some other major British cities have experienced a surge in gun crime in recent months, raising concerns that illegal weapons could filter from street gangs to extremists.



"We want to lessen the harm to our communities and to prevent the possibility of a marauding terrorist attack in the UK," said Lynne Owens, director general of the National Crime Agency, which leads Britain's fight against serious and organized crime.



"The UK's illegal firearms market is still geared mainly to criminals using weapons to protect against one another or to target one another, but we must be clear that one gun in the wrong hands in a public space is all it takes to cause devastation," she told reporters.



The threat level in Britain is officially set at "severe", meaning that an attack is considered highly likely, and a dozen attack plots have been thwarted since 2013.



Owens said one area of particular concern was the flow of illegal weapons from eastern Europe, especially the Balkans.



She said that while automatic weapons remained hard to find in Britain, there had been increases in the availability of Baikal pistols and converted weapons from eastern Europe.



Mark Rowley, the head of National Counter Terrorism Policing, said the availability of illegal guns and ammunition in Britain remained low compared with the United States and other European countries.



But he said police could not be complacent because guns were entering the illegal market through a variety of routes including the postal system and smuggling networks. He also said 800 legally owned firearms were going missing every year across the country.



Rowley said there had been instances where common criminals had drifted toward violent extremism, and this was one potential route for guns to reach radical groups.



"You do get gang members, criminals, people who are already angry, violent, difficult people causing problems in communities, who perhaps get given a clearer purpose for their violence by a terrorist ideology, whether they pick that up on the streets or in prison," he said.



"Those gang criminality links are an issue that concerns us. We have seen evidence of it potentially linking firearms into terrorism."