The controversy surrounding Mo Farah's coach Alberto Salazar shows no sign of abating after fresh details emerged from a US Anti-Doping Agency (USADA) report written last year.

The 269-page report was leaked to the Sunday Times by Russian hackers the Fancy Bears, with the newspaper reporting it was USADA investigators' belief that Salazar "almost certainly" broke anti-doping rules on drug infusions and may have endangered his athletes' health with unnecessary medication.

But on Friday, the BBC reported that the USADA dossier also said the 58-year-old American may have committed an anti-doping rule violation for possessing testosterone.

The World Anti-Doping Agency (WADA) rule 21.2.6 says athlete support personnel are prohibited from possessing banned substances without "valid justification".

The allegation that Salazar, who runs the successful Nike Oregon Project (NOP) running stable, used the steroid hormone while working with athletes was first made by the BBC's Panorama programme and American website ProPublica in 2015. That claim, and several others, prompted USADA to open an investigation which is ongoing.

Salazar has always denied any wrongdoing and told the BBC prior to publication of its latest report on the subject that he has been using testosterone, prescribed to him by a doctor, to treat a long-standing condition of hypogonadism. He also claims he has provided USADA with documentation to prove this.

According to the BBC the leaked USADA report refutes this claim, saying the paperwork does not show he suffers from the condition or that he needs testosterone replacement therapy.

The BBC says it asked Salazar whether he had provided further documentation to USADA subsequent to the leaked report from March 2016 being compiled, but did not receive a response.

Press Association Sport also attempted to contact Salazar on Friday but the coach did not immediately respond.

The USADA report - which it is understood was written in an attempt to force NOP doctor Jeffrey Brown to release the medical records of athletes he is alleged to have given unnecessary medication - goes on to state that an experiment Brown and Salazar conducted to see how much testosterone gel would trigger a positive dope test was "unlawful".

When the BBC and ProPublica first reported this incident, Salazar responded with a 12,000-word open letter that said the testosterone experiment, which used his own sons as "guinea pigs", was intended to protect his athletes from being sabotaged by a rival.

USADA's report, however, lists other potential rule breaches, including the alleged treatment of Farah's training partner Galen Rupp with testosterone medication from the age of 16, a claim Rupp has denied.

The persistence of these allegations, however, will inevitably lead to fresh calls for Farah to sever ties with Salazar, the man who helped him convert near-misses into gold medals. It will also heap pressure on British Athletics to end its partnership with NOP.

Speaking to reporters before the European Indoor Championships in Belgrade, British Athletics' performance director Neil Black said the governing body was "really confident" in the process it concluded last year which effectively cleared Salazar.

He also referred to USADA's response to the leak of its report on Sunday, which said all those under WADA's jurisdiction are "innocent and presumed to have complied with the rules unless and until the established anti-doping process declares otherwise".

Black said: "I think that is a perfectly reasonable stance and, in effect, that is our stance."

It is also Farah's stance, with the four-time Olympic champion posting a message on his social media accounts on Sunday that restated his denials of wrongdoing.

"I am a clean athlete who has never broken the rules in regards to substances, methods or dosages and it is upsetting that some parts of the media, despite the clear facts, continue to try to associate me with allegations of drug misuse," he wrote.

Salazar, who won the New York Marathon three times during his own running career, also has the support of his employers Nike.

In a statement released to the Press Association, Nike said: "Alberto has clearly and repeatedly refuted the allegations directed against him and his management of the Oregon Project.

"He has made it clear that the Oregon Project will never permit doping and all Oregon Project athletes are required to comply with the WADA Code and (athletics' governing body) IAAF rules."

Press Association Sport understands USADA's investigation into Salazar has not petered out, as many had believed after so long without news, but is now focused on an attempt to have Dr Brown struck off by his licensing authority, the Texas Medical Board.

Neither Brown nor the medical board has replied to requests for comment.