Defending world champion Dai Greene has been forced to pull out of the Diamond League meeting in Monaco.
Great Britain's Greene, 27, who won the 400m hurdles title in Daegu in 2011, was due to run on Friday but withdrew with an inflamed Achilles tendon.
The injury will cast doubt on the Welshman's ability to defend the title in Moscow next month.
Greene recently stated he was rediscovering his form before this latest setback.
The injury blow comes three weeks before the World Athletics Championships start and six days after his first victory of 2013 at the British Championships in Birmingham,
That race also saw Bath-based Greene run a season's best of 48.66 seconds - a confidence boost after a troubled campaign.
Greene was captain of Britain's athletics team at the London 2012 Olympics but just missed out on a medal when he finished fourth, and has been dogged by injury and illness since then.
He had a double hernia operation last winter and a virus hindered his preparations for the European Team Championships and Diamond League meeting in Birmingham last month.