Disgraced ex-Congressman Anthony Weiner repeatedly convinced an underage girl to get naked and fondle herself while he watched over the internet, federal prosecutors revealed in a court filing Wednesday.

Legal papers filed ahead of Weiner’s sentencing next week say the serial sexter took part in three video chat sessions via Skype during which the girl “made clear that she was not just a minor — she was, in fact, only ​​15 years old.”

“That did not stop Weiner,” the feds wrote.

“During the latter two Skype sessions, on February 18 and 23, 2016, and in a Shapchat communication on March 9, the defendant used graphic and obscene language to ask the Minor Victim to display her naked body and touch herself, which she did.”

Weiner, 53, also sent the girl an “obscene message” using an app called Confide, in which he described “what he would do to her, if she were 18,” court papers say.

“Part and parcel of these disturbing — and criminal — exchanges, the defendant also sent the Minor Victim adult pornography,” according to the feds.

​Prosecutors noted that ​Weiner “has acknowledged an interest in legal, adult, teen-themed pornography” which they said makes his claim that the girl’s age wasn’t a factor in his attraction ring “hollow.”

The feds also said that Weiner’s professed rehabilitation echoed his response to the sexting scandals that forced him to resign from Congress in 2011 and torpedoed his political comeback bid during the 2013 mayoral race.

“On prior occasions, as here, Weiner has followed the same pattern: he initially denied his conduct; he suffered personal and professional consequences; he publicly apologized and claimed reform,” prosecutors Amanda Kramer and Stephanie Lake wrote.

“Yet, he has, on multiple occasions, continued to engage in the very conduct he swore off, progressing from that which is self-destructive to that which is also destructive to a teenage girl — a minor the law recognizes as needing protection.”

The court filing further notes Weiner’s hypocrisy in having co-sponsored a 2007 bill aimed at forcing convicted perverts to list their email and instant-message addresses on the National Sex Offender Registry.

“In public remarks in support of the bill, the defendant recognized that “the Internet is the predator’s venue of choice today,” court papers say.

Weiner pleaded guilty in May to transferring obscene material to a minor and faces up to 10 years in prison when he’s sentenced Monday.

He and estranged wife Huma Abedin last week asked Manhattan federal Judge Denise Cote to spare him any time in the slammer so he can care for their son, but prosecutors Wednesday said Weiner deserves to get locked up for between 21 and 27 months.

“Although the defendant’s self-destructive path from United States Congressman to felon is indisputably sad, his crime is serious and his demonstrated need for deterrence is real,” the feds wrote.