Jon Brown and Liz Yelling have both been forced to withdraw from Great Britain's marathon team for August's World Championships in Helsinki.
Bedford runner Yelling has been battling a virus
Brown, who finished fourth in the event at the last two Olympics, says he has not recovered sufficiently from coming sixth in the London Marathon in April.
Yelling, who was the first Briton to finish the Olympic women's marathon, has missed training because of a virus.
Brown's withdrawal means the men will not compete in the team event.
However, Huw Lobb and Dan Robinson will still take part in the individual championship.
The women's squad of Paula Radcliffe, Mara Yamauchi, Hayley Haining and Debbie Mason are still eligible for the team competition with three athletes required to count.
The men's marathon takes place on 13 August and the women's event being held on 14 August.