The California welterweight lost a five-round decision to champion Georges St-Pierre on March 16 at Montreal's Bell Centre.
Since then, a video shot backstage at the weigh-in has raised questions about the Quebec athletic commission (Regie des Alcools des Courses et des Jeux), showing that it treats 170.9 pounds the same as 170.
Diaz was announced at 169 pounds at the weigh-in while St-Pierre was 170 — the welterweight limit.
The Diaz camp suggests the champion's exact weight may never be known.
"In the circumstances, Mr. St-Pierre remains legally and ethically obligated to fight Mr. Diaz at 170 pounds or else vacate the belt in favour of those prepared to fight at welterweight," Jonathan Tweedale, a Vancouver lawyer for the Diaz camp, said in a statement.
Traditionally fighters are given an extra pound at weigh-ins. So a welterweight can weigh in at 171 pounds, one about the limit. But that extra pound is not allowed in the case of championship bouts.
If fighters do not make weight, they are given an extra hour to do so. If they fail, the fight can go ahead but the overweight fighter owes his opponent 20 per cent of his purse.
In the UFC 158 backstage video, UFC vice-president Michael Mersch tells the Diaz camp what he has heard from the commission.
"Here, they're going to allow you and Georges to have an extra hour (top make weight)," Mersch says. "Just in case somebody doesn't make it.
"But the good news is, they don't count the decimal. If you're 170.2 it's 170. If it's 170.9 it's 170."
Mersch calls it "kind of an off-the-record type of thing."
"As long as he's under 171 (pounds), we should be good," he added.
A member of the Diaz camp then says: "That's a loophole … A Canadian loophole."
"The contracted weight for this fight was 170 pounds. 170.9 is not 170, anywhere in the world, for a title fight," the Diaz camp statement says. "There is no question what '170 pounds' means, in the bout agreement, as a matter of contractual interpretation.
"The Quebec Commission deliberately relaxed the rule in this case and, by its own admission, allowed their home-town fighter to 'make weight' even if he weighed more than the contracted weight."
The Quebec commission confirms that it does not take decimals into account in weigh-ins, but says that rule has been in place for some time and was not there to help St-Pierre.
"I wish to inform you that, during UFC 158, no contestants exceeded the weight determined in their contracts," Quebec commission spokewoman said in an email to MMAFighting.com.
"Currently, the Regie does take into consideration the maximum weight determined by contract when it carries out the weight-ins before a bout. However, our regulation on combat sports does not take decimals into account. Their consideration is a question of interpretation likely to be debated between the two parties under contract."
Tweedale says his complaint is about the Quebec commission's behaviour and not the UFC.
UFC vice-president Marc Ratner was not at the Montreal fight but said in an email that "my understanding was that both fighters weighed 170 or less" at UFC 158.
St-Pierre has proved to be a popular target.
Former champion B.J. Penn alleged the Canadian had illegally greased his body prior to their UFC 84 encounter.
The "Greasegate" allegations were never proved, although rules tightening who could apply Vaseline to fighters were tightened in the wake of the complaint.
As for the drug-testing complaint, the Diaz camp said no one observed their fighter provide a sample — suggesting the champion was also unsupervised.
Diaz, unlike St-Pierre, has flunked drug tests before. He has twice been suspended for testing positive for marijuana — something he says he uses medicinally to control social anxiety issues.
It has been a rough week for St-Pierre, who has also come under fire for walking into the arena wearing a gi and headband bearing the Japanese Rising Sun flag.
Korean UFC fighter Chan Sung Jung, known as the Korean Zombie, wrote an open letter to St-Pierre saying "for Asians, this flag is a symbol of war crimes."
Hyabusa, the Edmonton-based fightwear company that proved the gi, has apologized.
"We at Hayabusa have the utmost respect for culture and history and appreciate all of our customers worldwide," the company said via social media. "As such, we accept full responsibility for this design and are taking all complaints and comments very seriously.
"The gi worn by GSP will not be brought to market. In addition, we will be very conscious of this specific design element when developing future communication materials and products."
Meanwhile, the GSP camp and UFC are playing down reports that the welterweight champion is hurt.
"GSP has done some sprint training while on holiday. Despite injury report, everything is A-OK, says manager," tweeted Canadian UFC PR director Steve Keogh.
Trainer Firas Zahabi had told MMAFighting.com that St-Pierre suffered an Achilles injury before his March 16 title defence against Nick Diaz.
"I was worried it would tear during the fight," Zahabi was quoted as saying. "We had to cut his last sparring (session) short due to his injury. He had a week to rest it and then the fight."
The champion was fighting for the second time in four months after a lengthy layoff due to knee reconstruction.
He has said he wants to take some off before facing No. 1 contender Jonny Hendricks.
The UFC, meanwhile, said it was unable to confirm reports that St-Pierre will play Batroc the Leaper, a villain, in the upcoming "Captain America: The Winter Soldier" film, slated for release in April 2014.
St-Pierre's manager did not immediately respond to an email.