British newspaper The Mail on Sunday has reported that hurdler Rhys Williams and runner Gareth Warburton were permitted to arrange private testing of supplements in 2014.

This followed the pair testing positive for the banned anabolic steroid dienedione.

The positive tests were considered to have been caused by contamination of the Mountain Fuel Xtreme Energy drink.

A UKAD decision on the case said the drink had been sent to an independent laboratory, which confirmed anabolic steroid metabolites were present in the blackcurrant flavour. 

The Mail on Sunday reported that a representative of the athletes, supplement salesman Darren Foote, had supplements tested in a lab recommended by them but without any UKAD oversight.

The testing, conducted in September 2014, showed signs of contamination.

Cambridge Commodities, which manufactured the supplements, reportedly tested the batch in October at the same laboratory.

The manufacturer reportedly found that the supplements were not contaminated when they left their premises.

Williams' solicitor and Foote have reportedly suggested the packaging process was blamed for the contamination.

A UKAD spokesperson told The Mail on Sunday that it had not contacted Cambridge Commodities, so had not known the supplements were clear at source before or during the tribunal.

"After the case, UKAD became aware of a media report suggesting that other analysis had been conducted by Cambridge Commodities," UKAD said.

"UKAD has no record of having been provided with evidence of this analysis, since the conclusion of the case."

Williams, a European 400 metres hurdles champion, was banned for four months after demonstrating to the panel he had completed more checks on the origin of the supplements.

Fellow-Welshman Warburton received a six-month sanction.

Both athletes were ruled out of competing at the Glasgow 2014 Commonwealth Games during the case.

The pair were cleared of deliberately trying to cheat, with the UKAD decision saying the London 2012 Olympians had been "victims" but they had "brought this matter on themselves".

UKAD is already being investigated by the World Anti-Doping Agency (WADA) following reports it let British Cycling conduct its own probe into an athlete who tested positive for a banned substance in 2010.

The Mail on Sunday reported last week that the unnamed rider's sample had been found to contain traces of anabolic steroid nandrolone - defined as a "threshold substance" where a certain amount of it is required to trigger an anti-doping investigation.

According to the report, UKAD allowed British Cycling to carry out its own investigation, including urine testing of several athletes, into the abnormal levels of the substance in the sample.

Under the WADA Code, it would be up to UKAD to conduct such a probe.

UKAD also reportedly carried out the testing in a non-accredited WADA laboratory and was not party to any of the results.

A WADA spokesperson confirmed it had launched an investigation into the claims, which the global watchdog said was "of significant concern".