Police officers who carry a taser are more likely to be attacked than those without one because of a psychological theory known as the "weapons effect", a study has found.

Researchers at the University of Cambridge found City of London officers armed with the electroshock weapons were not only 50 per cent more likely to use force in the line of duty, but they were also twice as likely to be on the receiving end of attacks as unarmed colleagues.

Between June 2016 and June 2017, researchers allocated a taser-carrying officer to 400 frontline shifts and compared the number of assaults

Criminologists from Cambridge said the findings suggested tasers can trigger the "weapons effect": a psychological phenomenon in which sight of a weapon increases aggressive behaviour.

The weapons effect was first shown by psychologist Leonard Berkowitz in 1967, in a laboratory experiment involving the administering of electric shocks in the presence of a rifle.

Dr Barak Ariel, from the Cambridge Institute of Criminology, said: “For many, a weapon is a deterrence. However, some individuals interpret the sight of a weapon as an aggressive cue – a threat that creates a hostile environment."

Six physical assaults against police were recorded during shifts with taser-carrying officers, compared to just three on unarmed shifts.

Dr Ariel said: "We found that officers are more likely to be assaulted when carrying electroshock weaponry and more likely to apply force.

"The City of London Police rarely discharged Tasers during the study. Yet the very presence of the weapon led to increased hostility between the police and public.

"The presence of Tasers appears to provoke a pattern where suspects become more aggressive toward officers, who in turn respond more forcefully."

Study co-author Chief Superintendent David Lawes, from the City of London Police, said: "The use of Tasers have been a proportionate and sensible introduction to policing against a backdrop of unsophisticated terror attacks and an increase in violent crime across London.

"A number of other forces are interested in replicating the study to add to the evidence base and see whether the experiment produces the same results outside of London.

"Across our force, we will continue to use evidence to define how we target problems, which tactics we should use and how we can ensure policing is efficient and safer for both the general public and our officers."