A Metropolitan police sergeant who was filmed hitting a woman with a baton at the G20 demonstrations walked free from court today after a judge ruled he acted lawfully.

Delroy Smellie was suspended last year after video footage was posted on YouTube showing him back-handing a protester and striking her twice on the legs with his metal baton.

He was acquitted of assault by beating after a four-day trial in which his alleged victim, protester Nicola Fisher, declined to give evidence.

Smellie, from the Met’s territorial support group (TSG), a specialist public order unit, argued during his trial that he believed Fisher posed a threat to himself and fellow officers. He said he repeatedly struck Fisher, who was considerably smaller than him, after mistaking a carton of orange juice and digital camera she was carrying for weapons.

The district judge, Daphne Wickham, said there was no evidence that his use of the baton was not approved, correct or measured, adding that Smellie had a “mere seven seconds” to act, and other witnesses had feared for his safety.

A police officer who allegedly struck a woman during the G20 protests in London a woman is to be charged with assault, the Crown Prosecution Service said today.

A CPS spokeswoman said Sergeant Delroy Smellie would be charged with assault of Nicola Fisher and he will appear at Westminster magistrates court on 16 November. He faces up to six months in prison if found guilty.

Smellie, a member of the Metropolitan police’s territorial support group, was suspended from duty two months ago after footage emerged of him near the Bank of England, apparently hitting Fisher, 35, with the back of his arm.

He was also shown appearing to strike her on her legs with a baton as she attended a vigil for the newspaper seller Ian Tomlinson, who had died the previous day. She said the incident left her with severe bruising.