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Photos: Met Police

A 21-year-old British woman who disguised herself as a teenage boy to prey on girls online was sentenced to 8 years in prison on Friday.

Prosecutors say Gemma Watts created an online persona as Jake Watson, a 16-year-old boy, to groom minor girls on Snapchat or Instagram, Fox News reported.

Met Police said Watts used her online persona to strike up conversations with dozens of girls in private messages. She persuaded girls to meet her in various parts of England.

"She then went on to form physical relationships in which she spun a web of lies and deceit, giving her the opportunity to commit sexual offenses," said Phillipa Kenwrite, a detective who worked on the case.

Police were alerted by a Hampshire doctor who said a young patient was sexually assaulted by her teenage boyfriend.

Police say Watts sexually assaulted more than 50 girls. The girls believed they were in a relationship with a teenage boy and they didn't realize the deception until they were contacted by police.

"For some of these girls, it's one of their first relationships," Kenwrite said. "Her arrest, and now lengthy prison sentence, has prevented her from targeting further victims."