The Prime Minister has defended the Government's refusal to explain why one of the murderers of James Bulger has been returned to prison, as new claims emerged that the killer had a recent history of violence and drug abuse.

Gordon Brown told reporters yesterday that although the murder of James Bulger was an "abhorrent" crime, it did not justify making public the reason why Jon Venables, 27, has been recalled to custody.

He said he understood how the case "disgusts and offends the British people", but added: "What matters here is that the justice system is allowed to run its course and that justice is done, whatever wrongs are committed. The justice system must be allowed to take the action that is necessary. That is what people would expect and that is what is going to happen."

Reports have suggested that Venables was returned to custody more than a week ago after fighting with a work colleague and developing a drug addiction to ecstasy and cocaine since his release in 2001.

In the latest claims, the Daily Mirror alleged that the work colleague grappled with Venables before others intervened and pulled them apart. His alleged victim is said to have made an official complaint about the attack, which led to Venables being suspended from his job. This, the paper claimed, was what had triggered his recall.

It was also claimed that Venables has been hospitalised twice since being freed in 2001 after violent altercations. One report said he was stabbed more than two years ago, during a row after a man tried to chat up his girlfriend.

And it was alleged that in December 2008 he was arrested by police after being seen taking cocaine in an alleyway with another man. It was claimed he escaped with a caution.

The Ministry of Justice refused to confirm the claims. A spokesman would only say that Venables was back in prison after breaching the terms of his conditions of his licence.

By refusing to say how Venables had broken his parole, ministers provoked a bitter row with the family of the murdered two-year-old boy. The toddler's estranged parents, Denise Fergus and Ralph Bulger, were incensed and accused Justice Secretary Jack Straw of violating their rights as victims.

Mr Bulger, 43, insisted he had a right to know how Jon Venables broke the terms of his life licence. He asked: "What has the Government got to hide?" He said he found out about Venables' recall just hours before the story broke on Tuesday evening, despite Venables having been returned a week earlier.

Yesterday the Conservatives put pressure on the Government by requesting that more details about the case were disclosed. Ms Fergus, who is also fighting to find out why Jon Venables was recalled to custody, posted an internet message thanking members of the public for their support.