Dr Richard Spertzel claimed Dr Kelly was on a 'hitlist' in the final years of his life.
The former head of the UN Biological Section, who worked closely with Dr Kelly in Iraq in the 1990s, has written to Attorney General Dominic Grieve about the 'mysterious circumstances' surrounding the death.
The weapons inspector's body was found after he was unmasked as the source of a damaging BBC news report questioning the grounds for the Iraq war.
Dr Spertzel said: 'I know that David, as well as myself and a couple of others, were on an Iraqi hitlist. In late 1997, we were told by the Russian embassy in Baghdad. I had no idea what it meant but apparently David and I were high on the priority list.'
He said he and Dr Kelly were told that they were 'numbers three and four' on the list during an inspection trip in Iraq.
Concern over the cause of death has been mounting after it emerged in January that all medical and scientific records, including the post-mortem report and photographs of the body, were secretly classified for 70 years in 2004.