A serving police officer attached to Ealing Borough will face four years behind bars for sexual activity with a child.
PC James Evans was arrested in January following a report made to police, before he was charged with six counts of sexual activity with a child in July.
The victim was a girl under the age of 16 and the offences were said to have taken place between December 31, 2015, and January 28, 2016 while the officer was off duty.
PC Evans pleaded guilty to all six charges at Isleworth Crown Court before he was sentenced to four years in prison at the same court on Tuesday (August 30).
The 26-year-old has also been added to the sex offender's register for life.
Rt Hon Judge Robin Johnson said: "Before any sexual activity, you knew she was a vulnerable young girl. Nevertheless you allowed your own desire to overcome your better judgement.
"The reason why parliament and the public take this kind of offending so seriously is because the harm caused in these cases is incalculable.
"Her victim impact statement makes for sad reading, and I have no doubt this episode has caused her serious psychological harm."
Police said that now criminal proceedings have concluded, a misconduct review will take place.
Detective Chief Superintendent Matt Gardner, Commander of the Directorate of Professional Standards, said: "It is the duty of police to protect from harm, not to seek to cause it and this duty is especially important for the most vulnerable in our society including children.
"PC Evans' behaviour has fallen well short of the high standards that we expect. He has abused the trust placed in him by society and will now have to pay the penalty for his appalling actions in prison."