The internationally acclaimed architect appointed to chair the judging panel for the revamp of Glasgow's George Square has savaged the role council leader Gordon Matheson played in the debacle.

David Mackay described Matheson, who is currently being investigated by the police major crimes unit and the public standards watchdog, as "very unprofessional" and ''only concerned with getting one thing ... his own way".

Matheson announced the revamp had been abandoned minutes after his preferred design was rejected.

In a detailed statement to the Sunday Herald, Mackay, who designed the Barcelona Olympic Village and advised on the unification of Berlin, added: ''This [outcome] was not about the public interest, it was a strange demonstration of power by, I think, the oddest politician I have ever met in a lifetime of advising very powerful men and women, including mayors, the odd [Catalan] President and even Britain's Deputy Prime Minister [Labour's John Prescott].''

A senior council source dismissed Mackay's comments as "a petulant temper tantrum".

Mackay's devastating account of the days leading up to the £15 million project being scrapped earlier this year is another blow to Matheson's insistence that the proper procedure was rigorouosly followed.

The council abandoned the revamp after the competition judges voted against Matheson's choice and in favour of a rival scheme he opposed. The contest cost taxpayers £100,000 and the architects involved another £200,000.

Police and Scotland's ethics watchdog for councillors are now investigating claims that Matheson rode roughshod over a legally defined EU procurement process.

Mackay, a partner in MBM Architects Barcelona, was the original chair of the five-man judging panel for the design contest. However, at the first session, on January 14, Matheson replaced him by saying that, as council leader, he should be chair instead.

To keep the judging as objective as possible, the six shortlisted designs were anonymised. Matheson's favourite, by Burns + Nice, was known as number six, and the eventual winner, by McAslan & Partners, was number two.

Mackay said that despite being chair, Matheson abandoned all pretence of neutrality.

"I was shocked when Councillor Matheson firmly stated that the scheme number six was the public favourite and his too, without adding any reasoned arguments," he said.

"The favourite of the people of Glasgow, he assured us, was supported by the number of tweets and Facebook messages that had been received. He said it was therefore obvious that number six should be the winner.

"He was also very rude about scheme number two which he said was an insult to the people of Glasgow. None of his fellow jury members understood this statement."

But when the judges voted three days later, the McAslan design was the clear winner, and Matheson's favourite was ranked fourth.

Matheson "was obviously furious... and could not understand why the judges did not agree with him," Mackay said.

Mackay also contradicts Matheson's repeated public claim that the George Square redesign was dropped because of public opposition.

Instead, Mackay says that in private, Matheson spoke of public support for a change.

It was only after Matheson's preferred design was rejected by his fellow judges, and a design which Matheson detested was chosen as the winner, that the council leader suddenly announced the revamp had been cancelled because of public anger. It is now to get a modest "facelift" instead.

Mackay's statement says the judges were "all astounded since his [Matheson's] actions were very unprofessional.

"Never in all my dealings with a politician at all levels have I encountered one less willing to pause and listen or more determined to get his own way. Never, in my 25 years of judging international competitions."

Mackay said axeing the project "was a final indication that Councillor Matheson seemed only concerned with getting one thing, his own way.

"I now doubt if Councillor Matheson listened at any stage of the George Square judging, to anybody. If he did listen, perhaps he simply didn't understand what was being said."

A council insider last night claimed that, although Mackay was originally asked to chair the judging panel, the council decided in August that Matheson would in fact be chair, but this was never communicated to him or the organisers of the competition. Mackay was paid his £5000 chairman's fee to avoid a fuss, the insider said.

Mackay's comments chime with claims by the Royal Incorporation of Architects in Scotland (RIAS), the professional body which ran the contest, which last month filed a complaint with the Public Standards Commissioner suggesting Matheson repeatedly violated the Councillors Code of Conduct.

He faces claims of interfering in a legally defined procurement process, bias, attempted staff coercion and trying to "steer" the contest in favour of his preferred design.

If proven, the claims would end his career. It is understood Mackay's statement will be sent to the Standards Commissioner next week.

Police Scotland's Major Crimes and Public Protection Division and Audit Scotland are also investigating the contest.

The council's internal audit unit is also looking at senior staff working with Matheson, including Richard Brown, head of development and regeneration services

It emerged on Friday that Matheson will not face a leadership challenge at the Labour Group AGM on May 13. However, party insiders say the 46-year-old may yet be dragged down by the multiple inquiries into George Square.

Graeme Hendry, leader of the SNP opposition on the council, said: "The views of one of the most respected architects in the international community are damning on Councillor Matheson's objectivity, behaviour and judgment.

"It adds weight to the evidence that Councillor Matheson had little interest in the public reaction but scrapped the plans as he could not get his own way.

"He seems to have lacked any objectivity whatsoever when it came to judging. I hope the Standards Commissioner investigates thoroughly. He remains an embarrassment to Glasgow."

A council spokesman said: "There was no public appetite for a radical redesign of George Square.

"Whichever design had won, it would not have been implemented. Once again, no-one has provided any evidence to the contrary."