Floyd Landis has launched a stinging attack on Team Sky, claiming the idea of marginal gains and a zero tolerance policy is a facade and “just great PR”. The American, who turned informant after testing positive for drugs, scoffed at possible explanations for Chris Froome’s failed test and expects the fallout to be a death knell for Team Sky after eight years at the top.

The former US Postal Service rider won the Tour de France in 2006 before being told of a positive test for testosterone 72 hours later. He contributed to the downfall of Lance Armstrong with his testimony of widespread doping in cycling.

Landis fears history is repeating itself in the peloton, a theory he claims is supported by Froome’s adverse test. Froome and Team Sky deny breaking anti-doping rules and are seeking to explain why the four-times Tour de France winner had twice the legal level of the asthma drug salbutamol in a urine test during his Vuelta a España victory last September.

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The appeal process is expected to take at least a few more months. One possible explanation Team Sky may seek to support is that Froome’s adverse test was the result of too many puffs on his inhaler after the race. Another is that a kidney malfunction meant the 32-year-old secreted large quantities of salbutamol immediately before the drugs test.

Landis has no doubt it will eventually lead to a ban. “There is evidence that salbutamol can be performance enhancing if it’s used orally or intramuscularly,” he said. “It’s very difficult to get to the level Chris Froome showed by using an inhaler. If that will form his excuse I think it’s nonsense and I don’t think many buy it. He’s trying to defend himself because he has everything to lose. I feel sympathy for him but if he doesn’t face it now he will have to later.”

Team Sky were founded after decades of doping left the reputation of the sport in tatters but doubts have formed around Dave Brailsford’s team after Froome’s failed test, which came to light following a joint investigation between the Guardian and the French newspaper Le Monde.

They were already fighting scepticism inflicted by the mystery of a bag delivered to Bradley Wiggins at the Critérium du Dauphiné in 2011 and more recently with claims by the former Team Sky coach Shane Sutton that he viewed therapeutic use exemptions as a legitimate means to “find a gain”.

There was also an alleged delivery of banned testosterone patches to the Manchester velodrome addressed to Dr Richard Freeman, who was employed by Team Sky in 2011, which the medic claimed was sent by a supplier in error.

Landis does not conceal his amusement. “Sometimes random or coincidental things happen but I’ve got to be honest,” he said, laughing. “I find it very hard to believe a package of testosterone was accidentally mailed to a velodrome.

“We can take from what Shane has said they were at least pushing the limit with certain things. Now, with Froome’s failed test, if you take all those things together, there’s no defending that team. Any reasonable person would have more questions.

“There’s no belief in that zero tolerance system any more; that was never a real thing. It was just great PR about marginal gains and all these cute little sayings they thought up.”

Landis, now 42, lives predominantly in Colorado where recreational cannabis was legalised in 2012. He is out of cycling and owns a company, Floyd’s of Leadville, which retails marijuana and marijuana-infused products. He played a major part in Armstrong, his team-mate at US Postal, becoming one of the world’s most acclaimed athletes.

Facebook Twitter Pinterest Floyd Landis poses for a portrait in front of his office in Leadville, Colorado. Photograph: Daniel Petty/Denver Post via Getty Images

For four years after failing a drugs test Landis protested his innocence before becoming a whistleblower for a US Anti-Doping Agency investigation led by Travis Tygart but he feels the authorities have no real appetite for catching the cheats.

“Travis assured me his objective was to figure out the anti-doping system and eliminate the guys running the sport but that wasn’t done,” Landis said. “They were protected.

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“Wada is designed to protect the Olympic committee,” he added of the World Anti-Doping Agency. “They’re not designed to catch people using PEDs [performance-enhancing drugs] and here we are again. It’s been 11 years but the one thing that bothers me is I went through all that and accomplished nothing. I think Lance feels the same way and he has a right to. If you’re going to take us out and say: ‘These guys are the bad guys,’ you can’t just sit back on your hands and let things return to exactly the way it was.”

Froome has already committed to riding in the Giro d’Italia in May as he seeks to add another Grand Tour victory to the collection but Landis expects the Briton to be convicted of an anti-doping rule violation and to serve a ban. If this outcome unfolds he believes it will spell the end for Team Sky.

“When you have someone that high profile who suffers a ban it usually means the whole thing implodes,” Landis said. “If I was on the board of directors or an executive at Sky or any of the companies who sponsor them I would be long gone. At some point they have to make a decision that looks ethical.”