Paula Radcliffe has been forced to pull out of her comeback race following an 18-month absence in Manchester later this month due to illness.
The 37-year-old has been ruled out of the Bupa Great Manchester Run over 10 kilometres on May 15 because she has been suffering from a chest and throat infection.
The marathon world record holder had been set to compete for the first time since taking an extended break to give birth to her son Raphael.
"I'm really, really disappointed to have to pull out of the Bupa Great Manchester Run," said Radcliffe, who is building up for an assault on the Olympic marathon gold medal in London next year, the major title missing from her collection.
"I have missed racing so much and was so looking forward to running in Manchester in such a vibrant event.
"However, frustrated as I am, I know that the main focus for me is next year's Olympic marathon and achieving a qualifying standard later this year."
Radcliffe, who last raced at the New York Marathon in November 2009 when she finished fourth, admitted she could not take any chances with the illness.
"As an asthmatic I know that bronchitis, especially at altitude, needs to be treated with caution immediately," said the three-time London marathon winner, who regularly uses training camps in Albuquerque, New Mexico and Font Romeau in the French Pyrenees.
"Therefore it is vital to get fully healthy and have a strong comeback race as soon as possible."
Radcliffe, who gave birth to her second child Raphael at the end of September last year, is now set to return in the Bupa London 10,000 road race on May 30 when she will race on many of the roads that make up the 2012 Olympic marathon course.