BIRMINGHAM, England (AFP) - A "drained" Mo Farah withdrew from Sunday's Diamond League meeting in Birmingham, just a day after he insisted he would not be leaving his coach Alberto Salazar despite a BBC documentary alleging the latter had encouraged his athletes to use illegal substances.

The 32-year-old Somalia-born Farah, the double Olympic, world and European champion over the 5,000 and 10,000 metres, said he was "emotionally and physically drained".

"This week has been very stressful and taken a lot out of me," he said.

"I have not been able to focus properly on today's race and after the events of the last few days I feel emotionally and physically drained.

"I want to run well in the World Championships in Beijing and have decided it is better for me to go back to the US, seek answers to my questions and get back into training.

"I apologise to the people who bought tickets to come and watch me race and ask for your understanding at this time."

Farah had said at a press conference on Saturday that he was "angry" his name had "been dragged through the mud" and he had seen no evidence linking Salazar to doping. There was no indication then that Farah might withdraw from the 1,500m at Birmingham's Alexander Stadium.

"I'm not leaving Alberto, for the reason I've not seen any clear evidence," said Farah.

There was no suggestion Farah had done anything wrong and Salazar strongly denied all claims Wednesday's BBC documentary made, namely that he had encouraged athletes including America's Olympic 10,000m silver medallist Galen Rupp, a training partner of Farah, to use illegal substances.

Farah said: "I spoke to Alberto (on Friday), I got on the phone and said to him, 'Alberto, what's going on?' and he said, 'Mo, I can prove this to you - it's just allegations - I'll show you some evidence', and I said, 'Okay'.

"I'm really angry at this situation. It's not fair, it's not right. I haven't done anything but my name's getting dragged through the mud," added Farah, who did the double in the 5,000m and 10,000m at the 2012 Olympics, 2013 World Championships and last year's European Championships under Salazar's guidance.

"I'm a clean athlete. I'm against drugs 100 per cent and believe anyone caught should be banned for life.

"But it's not about me, so please - it's about Alberto. So let's put this on Alberto. Let's see if Alberto can prove to us... Till then, there's nothing we can do."