Inquiry chairman Lord Ranald MacLean was originally due to publish his Vale of Leven Hospital Inquiry report by the end of May 2011.
The latest delay comes after Lord MacLean requested an extension on the most recently proposed publication date of March 2014.
Health Secretary Alex Neil accepted the request, but frustratingly for the families of those who died no new date has been set.
The reason for the latest delay is to allow Lord MacLean time to issue warning letters to individuals or organisations likely to be the subject of criticism in his report.
A series of delays, including a period of illness which left Lord MacLean unable to continue his work, has frustrated the families of those who lost their lives as a result of the C diff (Clostridium difficile) outbreak at the West Dunbartonshire hospital.
The inquiry probed the treatment of 63 patients during the period between December 1, 2007 to June 1, 2008.
Of those 63 people, 31 patients died and C diff infection was identified as cause of death or a contributory factor to death in 28 cases.
Michelle McGinty lost her mother-in-law to C diff at the Vale and campaigned on behalf of the C diff Families Justice Group.
She said: "It is upsetting for families to get the news that this inquiry will be delayed for a fifth time.
"Families are now going through the heartache of the sixth anniversary of their loved ones' deaths while still receiving no answers to the public enquiry they fought so hard to get.
"It makes us even more angry this delay doesn't even have another timeline attached. Will we ever get the answers we so desperately need to finally finish grieving?"
Julie-Anne Jamieson, secretary to the inquiry, said the process of preparing warning letters, waiting for responses to the letters and then reviewing them "is a very complex and time consuming one."