David Hobbs, 67 was charged last year after emails given to the police by News International’s management and standards committee identified him as being the recipient of a payment of £750 for intelligence about Jon Venables, one of the two murderers of toddler James Bulger.
Venables had earlier been released under licence and given a new identity but was re-arrested over the possession of child pornography, leading to huge media interest.
Mr Hobbs committed the offence between 28 February and 2 April 2010 while working as an operations support grade officer at HMP Woodhill in Milton Keynes.
Another prison officer, Scott Chapman was sentenced to three and half years in December for selling stories about Venables to a number of newspaper including The Sun, The Daily Star and the now defunct News of the World for up to £40,000.
After the plea the presiding judge, Mr Justice Saunders gave Mr Hobbs unconditional bail while reports were prepared for the court, but advised him not to take that as a sign that he would avoid imprisonment.
Mr Hobbs is expected to be sentenced in February.