Christine Ohuruogu has withdrawn from the British Championships in Birmingham because of a virus.
The championships double as Olympic trials although it is not thought the former world and Olympic 400m champion's Rio place is at risk.
She tweeted: "Sorry guys, won't be running today, I have been fighting a virus and not ready to resume racing."
Ohuruogu, 32, won gold at Beijing 2008 and silver at London 2012, and was world champion in 2007 and 2013.
On Thursday, Olympic long jump champion Greg Rutherford also pulled out of the trials because of fitness concerns.
The 29-year-old made the precautionary move after suffering whiplash at the Birmingham Diamond League on 5 June.
BBC chief sports writer Tom Fordyce
"With the requisite two performances under the 400m qualification mark, both from last year, Christine Ohuruogu's place at her fourth Olympics is not immediately in doubt because of her late withdrawal from the trials.
"The selectors have a third discretionary place even if the first two women across the line in the final both have the times too, so the great championship performer - gold in Beijing, silver in London, twice the world champion - remains in the hunt for Rio."