Police officers carrying Taser electronic weapons are almost 50% more likely to use force and also more likely to be assaulted, a study has found.

Stun guns were de-holstered nine times during the year-long University of Cambridge study involving City of London police, and fired on two occasions. Officers carrying them were also more likely to use other types of force, such as handcuffing and CS spray, than those without stun guns.

The researchers say the findings suggest stun guns trigger the “weapons effect”, a long-established psychological phenomenon whereby sight of a weapon increases aggressive behaviour.

The results are likely to reignite the debate about the increased rollout of Taser electronic weapons over the past decade, which has been criticised by human rights campaigners.

The authors said the research did not aim to comment on whether stun gun use was a good thing but rather how they should be deployed. The researchers suggested the weapons should be concealed.

The lead researcher, Barak Ariel, said: “The very presence of the weapon led to increased hostility between the police and public … There are some people, probably the kind of people that come into contact with the police, that respond negatively to weapons, it provokes them … If the presence of weapons can lead to aggression by suspects, so its concealment should be able to reduce aggression and increase officer safety.”

The study, published in the Criminal Justice and Behaviour journal on Thursday, involved allocating 400 frontline shifts to an officer carrying a stun gun and comparing the results with an equal number of unarmed shifts.

Facebook Twitter Pinterest Ch Supt David Lawes, co-author of the study.

During the trial period, in June 2016-17, there were 5,891 incidents recorded by City of London police, who are responsible for policing the Square Mile. Use of force by officers carrying stun guns was 48% higher than among those who were unarmed. Even unarmed officers accompanying stun gun carriers used force 19% more frequently than those on stun gun-free shifts, researchers found.

Because there was no increase in injury of suspects or complaints against officers, the criminologists concluded suspects were instigating hostilities and police were responding forcefully.

Though the number of physical assaults against officers was small – six were recorded during stun gun-carrying shifts, compared with three on shifts without them – the researchers concluded the doubling was significant because such assaults were so rare.

Oliver Feeley-Sprague, Amnesty International UK’s programme director for military, security and police, said Taser electronic weapons were useful in “genuinely life-threatening situations” but the charity had “serious concerns about Tasers becoming the norm for day-to-day policing, and this study reinforces those warnings around the risks posed by the weapon”.

The study’s co-author, Ch Supt David Lawes of City of London police, said the force was reviewing the placement and exposure of stun guns, which are yellow and carried on the centre of the chest. “We still think it’s a really valuable tool but we are far more measured in our training and where we suggest the Taser should be carried,” he said.

The National Police Chiefs’ Council’s lead for less lethal weapons, Lucy D’Orsi, a deputy assistant commissioner, said she would study the findings. “Our experience of Taser is that they have a deterrent effect on violence and threats of violence,” she said. “In 85% of cases, the mere presence of the device is enough to bring an incident to a swift conclusion without the need for force to be used at all.”

Campaigners have expressed concern about the increased use of stun guns – up 51% last year to 17,000 incidents in England and Wales – particularly against vulnerable and black and minority ethnic people.