The rock singer Ian Watkins has received a 35-year sentence after admitting a string of sex offences involving children including the attempted rape of a baby.

Two women, known only as Woman A and Woman B – who are the mothers of children he abused – were sentenced to 14 and 17 years respectively.

Watkins was jailed by Mr Justice Royce for 29 years. He will serve at least two thirds of that before the parole board can decide if he should be released. If he is released early, he will serve the rest of the jail term on licence. But the judge stipulated he will serve an extra six years on licence on top of that, bringing the total sentence to 35 years.

Royce told Watkins: "Those who have appeared in these courts over many years see a large number of horrific cases. This case, however, breaks new ground.

"You, Watkins, achieved fame and success as the lead singer of Lostprophets. You had many fawning fans. That gave you power. You knew you could use that power to induce young female fans to help satisfy your insatiable lust and take part in the sexual abuse of their own children.

"Away from the highlights of your public performances lay a dark and sinister side."

Watkins looked blank as he was led from the dock. The sentence was greeted by shouts of "yes" from the public gallery.

During his sentencing hearing it emerged that the day after the former Lostprophets lead singer admitted his offences he told a female fan from prison that he was going to issue a statement saying it had been "mega lolz". He also told her he did not know "what everybody is getting so freaked out about".

Watkins, who has been on suicide watch in prison, told the woman, only identified as Samantha, that he had thought about telling the court: "Come on, it was not that bad; nobody got hurt." He said another tactic could be to "win them over with my charm" and claim: "I was off my head and do not remember anything."

The hearing was told that in a second conversation with the same woman on the following day he insisted that no baby was ever harmed.

Christopher Clee, prosecuting, detailed images found on Watkins's computer. Of a total of 90 images of child abuse, 24 fell into the most serious category. He also possessed 22 images of bestiality.

In mitigation, Watkins's barrister, Sally O'Neill, said her client's life had unravelled because of the pressures of fame and his drug addiction.

She said: "He was the singer of an extremely successful band that sold millions of records and the focus of considerable attention from fans. Fans who would do anything to attract his attention and once they had it do anything to keep it. It was 24 hours a day, seven days a week. He was bombarded with messages from fans trying to hit on him."

She said drugs played a "considerable part" in Watkins's offending. She explained his "mega lolz" comments as bravado, adding: "He was at a very low ebb and under considerable stress."

She said Watkins conceded that it was "probably his arrogance" that led him to believe he could live outside normal rules of morality. "He has perhaps, belatedly but nonetheless now, realised the gravity of what has happened," she said.

As Watkins begins his prison sentence, detectives said they would continue to investigate his activities.

They said they believed there were more victims and were liaising with forces across Britain, the international police organisation Interpol and the department for homeland security in the US. South Wales detectives have already travelled to the US and to Germany, where the band toured extensively, to try to establish if Watkins committed sex offences there.

The police, organisations that work with abused children and Watkins's former bandmates have all urged other victims to contact the authorities.

Watkins and two female fans in their 20s, who cannot be named, had been due to stand trial last month at Cardiff crown court for a total of more than 20 offences, including allegations involving the women's children, a boy and a girl. He had denied the accusations and loyal fans had attended court to show their support.

But at the last moment – after jury members had been warned they would have to examine some very disturbing images and arrangements had been made for them to receive counselling after the trial – Watkins and the women pleaded guilty to almost all the charges.

Watkins, whose former band has sold around 3.5m albums, admitted 13 charges over five years including attempting to rape one of the children and conspiring to rape the other. The court was told that the two women sexually abused their children at his behest and were prepared to make the children available to him for sex.

The prosecution said the attempted rape happened while Watkins was staying at a hotel in west London shortly after appearing on BBC Radio 1.

When police investigated Watkins, who is from Pontypridd in south Wales, they found a "secret" computer disc. GCHQ experts helped police get access to the material on the disc, which featured videos of sexual abuse, including the attempted rape.

Watkins, a user of crack cocaine and crystal meth, maintained he could not remember the incident but the court was told this could have been because he was high on drugs. It emerged he had also discussed forcing his victims to take drugs – and one of them was found to have been "exposed" to crystal meth.

Clee, the prosecutor, branded Watkins a "determined and committed paedophile". The court heard of one exchange in which a woman offered him a "summer of child porn". He replied: "Hell yes, baby."

Watkins's barrister claimed that Woman A and Woman B were equally to blame for the abuse that was carried out.

But Woman A's barrister, Jonathan Fuller QC, said his client was an impressionable 17-year-old when she met Watkins for the first time.

"She was corrupted by him," said Fuller. "He darkened her world with drugs and even injected her with heroin. She sacrificed her own moral compass so she could sustain a relationship with a man she was obsessed with.

"She was a girl doing her A-levels. He was in the limelight and a rock star. She was vulnerable and exploited."

Christine Laing QC, for Woman B, said her client was a "very immature young woman" suffering from an undiagnosed personality disorder and postnatal depression when she first spoke to Watkins. Laing told the court how Watkins had told Woman B: "You and your daughter now belong to me."

South Wales police – who have codenamed their investigation Operation Globe – said detectives had spoken to witnesses around the world. They said two young victims were now being cared for in places of safety, but the force would continue to question Watkins in case there were other victims.

Before the sentencing, Detective Chief Inspector Peter Doyle said he believed there were further victims.

Co-founded by Watkins in Pontypridd in 1997, Lostprophets released five albums. The other members of Lostprophets announced the band was splitting up last month. They have made it clear they knew nothing of Watkins's offending.

After the singer's conviction, HMV removed the band's music from sale. Rhondda Cynon Taf council agreed that paving stones bearing his lyrics in Pontypridd should be removed.