Manchester City defender Richards, 22, had reported sick after his club's Premier League defeat at Sunderland on Saturday.
He was given extra time to recover but failed to do so in time to make Wednesday's flight to Portugal for Friday's penultimate Group 9 clash.
Richards' withdrawal follows those of Jack Rodwell, Andy Carroll, Victor Moses, Henri Lansbury and Alex Smithies.
Midfield star Rodwell and forwards Carroll and Moses all pulled out with ankle injuries, while Lansbury had a groin problem and Smithies a dead leg.
The call-offs were bad news for England Under-21s, who need to win both Friday's match and Tuesday's game against Lithuania - and hope Greece do not win in Macedonia on Saturday - to guarantee a place in next summer's finals.
Smithies was replaced in the squad by Middlesbrough's Jason Steele for the Portugal game but could yet rejoin the party for Tuesday's match in Colchester.