James Bulger, two, was beaten to death on a railway line in Liverpool, northwest England, in 1993 by Jon Venables and Robert Thompson, who were aged just 10 at the time.

The pair, who became hate figures in Britain, were later given new identities and released on licence in 2001.

A string of media reports suggest Venables, now aged 27, has now been taken back into custody over alleged child pornography offences but has not yet been charged.

While confirming that an investigation into Venables is under way, ministers have refused to officially reveal the nature of the "serious" claims, saying it could prejudice any future trial.

Denise Fergus, James's mother, accused officials Monday of having broken a promise to tell her if either of the young killers broke the terms of their release.

"My head has been all over the place. I don't know what he has done," she told ITV television in what was her first TV appearance since the news broke.

"I have had sleepless nights and I am not eating again -- I have had to pull my kids out of school.

"It is just one massive rollercoaster again for me and I can't believe that they are putting me through this."

Amid intense media pressure to reveal more details of what Venables has been accused of, Justice Secretary Jack Straw told the House of Commons he had considered doing so, but decided against it.

"This is because the police and the director of public prosecutions have advised that a premature disclosure of information could undermine the integrity of the criminal justice system," he said.

"Our motivation is solely to ensure that extremely serious allegations are properly investigated and that justice is done.

"No-one in this country would want anything other."

Later Monday, the judge who granted Venables anonymity warned he could be murdered by vigilantes if his new identity is revealed.

Addressing lawmakers, Anne Butler-Sloss stressed "the enormous importance of protecting his anonymity now and if he is released because those who wanted to kill him in 2001 are likely to be out there now."

Butler-Sloss has retired as a judge and is now a lawmaker in Britain's upper house of parliament.

Straw had seemed to confirm some of the allegations against Venables in an earlier interview Monday.

"What I want to do is to look at what has already been made public... to look at that and then to make a judgment about whether there is information that, given it is now out in any event, we could now confirm," he told BBC radio.

The family's local lawmaker, Labour's George Howarth, told the Commons they were "very, very distressed indeed".

"At the moment, the problem is there's so much speculation that's only adding to the distress of the family," Howarth added.