JAMES Bulger's killer Jon Venables was thrown back in jail over an allegation that he committed a sickening sex crime, The Sun can reveal.

The 27-year-old is expected to be brought before a court in the near future to face charges over it.

The Sun has discovered exact details of the offence, that will horrify the nation.

But government lawyers last night threatened to gag us with an injunction if we revealed it in full

If Venables is found guilty of the offence, it will represent a staggering failure in his close supervision by the Justice Ministry's Probation Service.

Aged ten, Venables and pal Robert Thompson were caged for torturing and battering two-year-old James on Merseyside in 1993. They were released on special licence amid huge controversy in 2001 with new identities and lives.

Murdered ... James Bulger

They had to abide by a series of strict conditions, which would be monitored very closely by probation officers.

Since Venables was picked up by cops last week, the Government has covered up why.

Mother ... Denise

Despite a huge public outcry - including 23,245 Sun readers signing a petition - they have refused to reveal what he has done.

Late last night, Treasury solicitors acting on behalf of the Government's top legal adviser, the Attorney General, demanded we withhold any information about Venables' alleged offence.

They argued that publishing full details of it would lead to his identification and potentially prejudice any criminal trial.

Three out of four people want the reason for Venables' return to be confirmed, even if it puts him "at risk," a YouGov poll for The Sun found.

PM Gordon Brown rejected pleas from James' parents Ralph Bulger and Denise Fergus to reveal why the killer is behind bars.

Last night Justice Secretary Jack Straw agreed to meet Denise.

We told yesterday how Venables' cover has been blown in jail.

Earlier inaccurate reports suggested Venables had been recalled to prison over a punch-up with a workmate and taking drugs.

But The Sun understood that only an allegation of an extremely serious crime could have resulted in his being banged up again.