WINCHESTER, England—A young British woman who posed as a teenage boy to deceive potentially dozens of girls into having sexual contact with her was jailed for 8 years on Friday.

Gemma Watts, 21, created an online persona as Jake Waton, a 16-year-old boy, and used social media to strike up relationships with girls as young as 14. Police described her male alter ego as very believable.

Watts pleaded guilty in November to 7 charges relating to 4 victims aged 14 and 15, although police believe she was a prolific predator and that there were likely to be many other unidentified victims.

The charges included assault, sexual assault and meeting a child following sexual grooming. Watts was sentenced to 8 years in prison at Winchester Crown Court in southern England.

She would start her grooming by liking girls' profiles on Snapchat or Instagram, then sending them complimentary messages. Soon she would be calling them "Babe" or other pet names.

Her own profile as Jake Waton featured posts about skateboarding and photographs of herself, dressed in baggy sportswear, with her long hair tied up in a bun and hidden under a cap or hood.

Having persuaded her victims to meet her, Watts travelled by train to their homes in various parts of England. The girls believed they were in genuine relationships with a teenage boy. In several cases, there were multiple encounters over a period of months.

She met the parents of some of her victims and had conversations with them. They too were taken in.

Phillipa Kenwright, one of the detectives involved in the case, said the victims were unaware they had been deceived until they were contacted by police, and it had been devastating for them to learn the truth.

"It's been life-changing for all of the victims involved," Kenwright told reporters in a briefing ahead of sentencing. "For some of these girls it's one of their first relationships."

The case has not previously been made public, and Kenwright said she expected that once the details were in the media, more victims would come forward. She said her personal estimate was that there might be between 20 and 50 unidentified victims.

Watts, who left school at 16 and lives with her mother in north London, first came to the attention of the police in April 2018. Despite knowing she was under investigation, she continued her activities as Jake for many more months.

Kenwright said Watts had expressed no remorse and given no explanation for her actions.