Froome pulls out of Liege at last moment
British cyclist Chris Froome competes in the USA Pro Cycling Challenge in Breckenridge, Colorado, on August 21, 2013 - by Garrett Ellwood
Briton Froome has not raced since the end of March when he finished an underwhelming sixth in the Tour of Catalonia.
"Unfortunately Chris has been suffering from a mild chest infection," said Team Sky doctor Richard Freeman in a statement on the team's website.
"After an examination this morning (Sunday) we concluded that the best course of action would be to withdraw him from Liege-Bastogne-Liege as a precautionary measure, which will allow him the best chance of recovery ahead of the Tour de Romandie."
Froome was forced out of the Tirreno-Adriatico race in March due to a back problem before making his comeback in the Tour of Catalonia, where he was short of his best form.
The 28-year-old had started the season in fine form, winning the Tour of Oman in February for the second year in a row, but his season has since then been disrupted by injury and illness.
The Tour of Romandie, in which he is the reigning champion and which begins on Tuesday, is a crucial step in Froome's preparation for July's Tour de France defence.
After that he is due to race in the Criterium du Dauphine in June, where he is also the title-holder and which is considered one of the best gauge's of a rider's form before the Grand Boucle.
Briton Pete Kennaugh also pulled out of the Liege race, one of five 'Monuments' in cycling, due to an unrelated illness, Sky said.
It left Sky with just six riders for the Sunday's 100th edition of 'La Doyenne' (the oldest race) instead of eight, with Australian Richie Porte leading the way.
Another rider who didn't take the start was Colombian outsider Carlos Betancur.
The 24-year-old AG2R rider has been suffering from illness recently, forcing him to abandon during both the Tour of Catalonia and Tour of the Basque Country.
He was fourth in this race last year, his breakout season, and showed he was in fine form last month by winning Paris-Nice.
It wasn't a great day for Tour winners as Luxumberger Andy Schleck pulled out after around 175km, with his Trek team saying he was still suffering pain in his knee from a crash at last Sunday's Amstel Gold Race.
The 2010 Tour winner started all three Ardennes Classics this week but failed to finish any of them.
And world champion Rui Costa's poor showing in the Ardennes continued as he was forced to abandon 75km from the end after a heavy crash in the peloton.
The Portuguese Lampre rider came into these three races as one of the favourites but after finishing 17th at Amstel was a lowly 53rd at Wednesday's Fleche Wallonne.