The witness, who can be identified only as "Person X", was expected to provide major testimony at the trial of Cody Rae Haevischer and Matthew Johnston.
Both Haevischer and Johnston have pleaded not guilty to charges of conspiracy and six counts of first-degree murder.
Now in the latest details to emerge from the trial, the court has revealed "Person X", formerly a Red Scorpion gang member, was actually prohibited from testifying in a decision made last summer.
However, that information could only be revealed publicly now, after Justice Catherine Wedge dropped a small part of the sweeping bans in the case.
"On Aug. 14, 2013, I ruled that an important witness, Person X, cannot be called by the Crown in the trial of this matter," stated Justice Wedge. "His evidence is inadmissible."
Justice Wedge said a confidential informer made statements about "Person X". The nature of that information has not been revealed.
It is not known what impact the loss of "Person X's" testimony will have on the trial of Haevischer and Johnston. They had been standing trial along with alleged gang leader Michael Le, but he pleaded guilty in November to conspiracy to commit murder.
Two other men also charged in the case, Jamie Kyle Bacon and Sophon Sek, will be tried separately at a later date.
Six people were killed in the attack in a Surrey, B.C., high-rise in 2007.
Four of the victims, Ryan Bartolomeo,19, brothers Michael Lal, 26, and Cory Lal, 21, and Edward Narong, 22, were described by police as having criminal lifestyles.
Two others, Chris Mohan, 22, and Ed Schellenberg, 55, were determined to be innocent bystanders.
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