Ibrahim Darraji, who leads Syria's own investigation into the killing, on Monday said the new testimony from Kurdish former intelligence agent Hussam Tahir Hussam spelled the collapse of last month's findings by UN investigator Detlev Mehlis. The report prompted a Security Council resolution against Damascus.
Darraji's attack on the commission came a day before Mehlis's team was due to hold its first interviews with senior Syrian officials at UN offices in Vienna, ending a prolonged wrangle over the venue for the long-awaited interrogations.
"From a legal point of view, the Mehlis report has collapsed," Darraji told reporters at a joint news conference with the purported witness in Damascus.
"It was based on the testimony of two key witnesses - Muhammad Zuhair as-Sadiq, who is now jailed in France, and Mr Hussam," he said, standing alongside Hussam.
"The ball is now in the Mehlis commission's court - they based their findings on the statements of one person and he has now set the record straight."
In the lengthy interview broadcast on Sunday evening, Hussam told state television that he had testified against the brother and brother-in-law of President Bashar al-Assad only under duress.
"Maher al-Assad and Assaf Shawkat were the main officials in their sights .. they asked me to speak out against them and I said that they were the ones who ordered the murder," he said.
He said he regretted making what he described as the "entirely false" statements that he had given "under duress and for astronomic sums of money".
Lebanon's Interior Ministry, now controlled by opponents of Syria, had offered him $1.3 million, Hussam said.
He did not say what the duress consisted of or whether he had accepted the alleged cash offer.
A statement from the UN commission confirmed that Hussam had testified before it but added that he had volunteered his testimony and had expressed fear about the repercussions from the Syrian authorities.
"From a legal point of view, the Mehlis report has collapsed"
Head of Syrian investigation into the assassination of al-Hariri
"Hussam first approached the UN International Independent Investigation Commission at the end of June 2005 and identified himself as a former Syrian intelligence officer in Lebanon," the statement said.
"In his witness statement signed and dated 1 September 2005, Mr Hussam stated: 'I am here voluntarily to give a statement to the UNIIIC. I have not been threatened or forced to come here, nor have I been offered any promises or incentives to do so."
"I understand that by giving knowingly false information in this witness statement I may commit a crime against the laws of the Republic of Lebanon," the statement quoted him as saying.
The commission insisted it had "never offered or provided" any incentive for testimony and added that, according to Hussam's statements, it was the Syrian authorities that he was afraid of.
"On several occasions Mr Hussam expressed fear to UNIIIC that he and his family could be harmed by Syrian security elements," its statement said.
The UN commission was due to question five senior Syrian officials in Vienna on Tuesday. Its interim findings implicated them in al-Hariri's murder.