The Royal Incorporation of Architects in Scotland (RIAS), which ran the design contest, says council leader Gordon Matheson was at the heart of a "debacle" which brought the city into "significant disrepute", and cost taxpayers £100,000 and architectural practices a further £200,000.
In the first authoritative insider account of the shambles, which ended with Matheson scrapping the revamp of the square just minutes after a winning design was chosen, RIAS reveals the leader:
l Personally drove the redesign process throughout
l Insisted he was chairman of the judging panel, usurping the role of the official chairman
l Deeply resented the winning design, which he considered "disrespectful" to Glasgow
l Rejected RIAS advice on good practice
l Temporarily "abandoned" the judging process, leading to it being lampooned in the media
l Directly contributed to a "very significant waste of public and private resources"
The SNP and Tories on the council last night urged Matheson to apologise over the affair.
The RIAS report adds to the pressure on Matheson ahead of the council's Labour group AGM in May, when he could face a leadership challenge.
In a devastating conclusion, the report adds: "It appears that Councillor Matheson had selected his own winner at the outset, and reasoning by a very experienced group of judges did not persuade him otherwise."
A Glasgow City Council spokesman refused to comment.