An investigation is under way after a photograph of Charlotte Mulhall holding a knife to a male inmate’s neck became public.

Charlotte Mulhall and her sister Linda are serving sentences for killing Fareh Swaleh Noor whose dismembered remains were found in the Royal Canal in Dublin in 2005.

The inquiry began after the photograph appeared in a number of newspapers.

The Labour Party has issued a statement calling on Minister for Justice Dermot Ahern to explain the incident.

The statement says that the photograph raises serious questions about the adequacy of security procedures within the prison system.

It says the incident raises a number of crucial questions such as how a prisoner convicted of murder was apparently in possession of a knife and also how male and female prisoners were apparently socialising together.