Police Scotland has spent three years probing nine accusations of criminality against the Crown Office, police and forensic officials who worked on the investigation into the 1988 bombing in which 270 people died.
The allegations include perversion of the course of justice and perjury.
The force is due to release a report imminently and campaigners who believe Abdelbaset al-Megrahi was wrongly convicted of the bombing argue the findings should not be handed to the Lord Advocate and Crown Office, but to an independent prosecutor instead to consider what, if any, action should be taken.
Brian McConnachie QC will join Dundee University law professor Alan Page, co-convener of the Scottish Greens Patrick Harvie and John Finnie MSP at a Justice for Megrahi campaign group conference in Edinburgh on Wednesday.
Prior to the conference, Mr McConnachie said: “Having declared the allegations to be defamatory, unfounded, false and misleading, it is in my opinion impossible for any decision of the Lord Advocate arising out of the allegations to be seen to be impartial, objective or unbiased.”
Mr Harvie said: “While the Lord Advocate rightly enjoys independence and freedom from political influence when considering police reports into criminal acts, that consideration must be seen to be truly independent and not coloured by any bias or prejudice whatsoever.”
Megrahi, the only person convicted of the Lockerbie bombing, died in 2012 having been released from a Scottish jail in 2009.
The Pan Am flight was on its way from London to New York when it exploded above Lockerbie on the evening of December 21 1988, killing everyone on board and 11 people on the ground.