Mo Farah has withdrawn from Sunday's Diamond League meeting in Birmingham following doping allegations made against his coach.
Alberto Salazar has been accused of encouraging Galen Rupp, Farah's training partner, to violate anti-doping rules since the US 10,000m record holder was 16. Salazar has strenuously denied the claims, aired on a BBC Panorama programme.
Farah, coached by Salazar since 2011, told a packed press conference on Sunday morning he was too "emotionally and physically drained" to compete. He plans to return to his base in America and "seek answers" from Salazar.
Farah, the 5,000m and 10,000m Olympic and world champion said: "This week has been very stressful and taken a lot out of me.
"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."
There is no suggestion Farah has done anything wrong, but he said he is "angry" at his name being "dragged through the mud".
Salazar, Rupp and Farah's agent, Ricky Simms, were made aware of the allegations one month ago. Farah and his agent were seen together when he won the 10,000m in Oregon last weekend, before the programme was aired.