Mo Farah is facing questions over his credibility after it emerged he repeatedly denied to US anti-doping investigators that he was given injections of a controversial supplement – only to change his account of what happened shortly afterwards.

Emails obtained by BBC Panorama also show senior UK Athletics figures debated whether giving Farah four injections of L-carnitine two days before he ran the 2014 London marathon was ethical and in the “spirit of the sport”.

Four men – the UK Athletics chief medical officer at the time, Dr Rob Chakraverty, the head of endurance running Barry Fudge, the former UK Athletics performance director Neil Black, and Farah’s former coach Alberto Salazar – were present when the amino acid was administered to Farah in a bid to boost his performance before his first attempt at 26.2 miles.

However, the programme claims that when Farah was drug tested six days after the injections, on 17 April 2014, he did not record L-carnitine on his doping control form as he was required to do. The programme also revealed that a year later, when he was questioned by Usada officials in London for nearly five hours, he also initially denied receiving the supplement.

According to transcripts obtained by the BBC, Farah was asked by Usada’s investigators: “If someone said that you were taking L-carnitine injections, are they not telling the truth?”

Farah replied: “Definitely not telling the truth, 100%. I’ve never taken L‑carnitine injections at all.”

He was then asked: “Are you sure that Alberto Salazar hasn’t recommended that you take L‑carnitine injections?”

Farah responded: “No, I’ve never taken L-carnitine injections.”

He was asked again: “You’re absolutely sure that you didn’t have a doctor put a butterfly needle – into your arm – and inject L‑carnitine a few days before the London marathon?”

Farah replied: “No. No chance.”

While L-carnitine is a naturally occurring amino acid, the World Anti-Doping Agency has strict rules governing its use – with a maximum of 50ml being allowed every six hours. Salazar was found guilty of two violations of these in respect of other people when he received a four-year doping ban in October.

According to Panorama, minutes after leaving the interview, Farah then met Fudge – who had been interviewed by Usada the day before – and told its investigators that he had got the L-carnitine from Switzerland from a contact of Salazar’s.

He then returned to the interview room as the investigators were packing up and gave a different account of his L-carnitine use. The transcript shows Farah telling Usada: “So I just wanted to come clean, sorry guys, and I did take it at the time and I thought I didn’t.”

The Usada investigator then says: “A few days before the race – with Alberto present and your doctor and Barry Fudge – and you’re telling us all about that now but you didn’t remember any of that when I kept asking you about this?”

Farah responds: “It all comes back for me, but at the time I didn’t remember.”

When questioned by the Times about the incident, Farah said he had initially forgotten about the injections and the name of the supplement before being reminded by Fudge.

“I finished the interview and then realised ‘shit’, I just remember having that injection,” he explained. “What was the name of it? L-carnitine. And I was like ‘shit’.”

In 2017 a parliamentary select committee was told that Farah had been given, in total, 13.5ml of L-carnitine – well under the permitted dose – but that Chakraverty had been censured in his appraisal for “his inexcusable conduct” in not recording it in any official records. An apologetic Chakraverty, now the England football team doctor, insisted he had been too busy dealing with 140 other athletes under his wing to record the dosage.

Speaking to the Times, Farah expressed his frustration that the procedure was not properly recorded by Chakraverty. “I wish at this point I’d never taken it because it never did anything for me. I ran shit that day.”

While Farah declined to be interviewed by Panorama, a statement from his lawyers underlined he had done nothing wrong. “Mr Farah understood the question one way and as soon as he left the room he asked Mr Fudge and immediately returned to clarify and it is plain the investigators were comfortable with this explanation,” the letter read.

“It is not against [Wada] rules to take [L-carnitine] as a supplement within the right quantities. He is a human being and not a robot. That is relevant – if in fact something was missed from the form. Interviews are not memory tests.”

On Saturday Farah admitted to the Times he had not been truthful when asked at the 2016 Olympics whether he knew the controversial coach Jama Aden, who is under investigation by the Spanish police.

“When I got asked and I said in that press conference [in Rio] and said: ‘Yeah, I know him but I don’t know him’, that’s not the reality,” he told the newspaper.

Panorama has also obtained emails showing UK Athletics officials initially expressed concern about whether the injection was safe and within the “spirit of the sport”.

On 6 April 2014 Fudge wrote: “Whilst this process is completely within the Wada code there is a philosophical argument about whether this is within the ‘spirit of the sport’.”

Black also admitted to “a degree of discomfort”, adding: “Should we really be trialling this process so close to the London Marathon? That’s before we even think about the spirit of sport.”

Chakraverty also seemed concerned about possible “side‑effects”, also writing: “I understand [Salazar] is keen but we should be asking him to follow this advice.”

In a statement to Panorama, Chakraverty said: “I have not contravened any rules, and have always acted in the best interests of those I treat. The evidence I provided to [MPs] was an honest account – including an acknowledgement that my usual standard of record-keeping slipped due to heavy work commitments and travel. The GMC reviewed this and concluded that the case required no further action.”

UKA, meanwhile, said: “A small number of British athletes have used L‑carnitine and, to our knowledge, all doses and methods of administration have been fully in accordance with Wada protocol. The dosage provided to Mo Farah was well within the 50ml limit permitted. Full and honest accounts of the process were given in all forums. Any suggestion to the contrary is misleading.”

Black, who left UKA last year, told the Guardian he had “nothing to say at this point” while Fudge also declined to comment.

Salazar, who is appealing against his four-year ban for doping offences, said in a statement: “No Oregon Project athlete used a medication against the spirit of the sport. Any medication taken was done so on the advice and under the supervision of registered medical professionals.”

Mo Farah and the Salazar Scandal is on Panorama on Monday 24 February at 8.30pm, BBC One and iPlayer.