He made a last-minute change of plea before his trial started, sparing the jury from seeing what the judge called "extremely graphic and distressing'' video footage.
Watkins plotted the abuse of a one-year-old child in messages with the mother, including one in which he said he wanted a "summer of filthy child porn'' and to take things "to the next level''.
He then recorded camcorder footage of himself performing a sex act on the child during a drug-fuelled binge, after arranging to meet the child's mother in a London hotel room.
Detectives found the footage uploaded to an online "cloud'' storage facility.
Watkins then persuaded another female fan over the Skype network to abuse her own child. The split screen footage shows him referring to the woman and her baby as his "slave duo''.
They also discuss making the child have sex with animals and take drugs as it grows up.
Police raided Watkins's house and found films of him performing sex acts with under-age girl fans.
A laptop seized from his home had to be cracked by the British government's electronic eavesdropping system GCHQ, the court heard.
Watkins was originally charged with rape but insisted actual sex with the child did not happen.
He pleaded guilty to 11 charges on Tuesday, including two of attempted baby rape and others involving possessing and making images of sexual abuse and conspiracy to rape.
Prosecutor Christopher Clee QC told the judge that both of the attempted rape admissions were accepted.
He acknowledged that there was an argument regarding what key video evidence seized from Watkins's laptop actually showed.
"From the footage, there is an argument as to whether the full offence is made out," he told judge Mr Justice Royce.
"If it is made out, it is minimal. There is so little difference between the full attempt and the attempt as to make no difference ... We are prepared to accept the pleas as tendered."
He also told the judge that the prosecution had in mind the effect on the jury of watching the explicit video footage.
Previous hearings discussed arranging counselling for jurors who might need it.
Watkins will be sentenced on December 18.
Lostprophets formed in 1997 and had a chart-topping album in 2006 with Liberation Transmission. They disbanded after Watkins' arrest.
- Independent with AP
