Richie Porte withdrew from the Volta a Catalunya on Tuesday, suffering from the lingering effects of the illness that saw him quit Tirreno-Adriatico earlier in March.
It is a further setback for the Tasmanian's race programme and training schedule ahead of his assault on the Giro d'Italia in May, where he is set to lead Team Sky for the first time in a Grand Tour.
"I'm obviously not 100 percent," said Porte. "After my illness at Tirreno I went home and just haven't quite been able to get over it.
"Training is one thing, but when you come to the race there's nowhere to hide. Unfortunately today I had to stop but I think long-term I'll be able to go back, get healthy, recover and re-start my season."
Porte's withdrawal from Volta a Catalunya leaves Chris Froome without one of his key helpers during the race's mountain stages on Wednesday and Thursday.
Sky's spring stage races have so far been heavily affected by illness and injury. Froome won the Tour of Oman in February, but subsequently missed out on riding Tirreno-Adriatico due to back problems. Porte replaced him in the Italian stage race, but he too withdrew. In the same week, Geraint Thomas was sitting in second-place overall in Paris-Nice when he crashed out of the race.
Thomas returned to racing at Milan-San Remo on Sunday (March 23), where team-mate Ben Swift placed third. Thomas continues his race programme with Sky's Classics squad at the E3 Harelbeke on Friday and Ghent-Wevelgem on Sunday.
Team Sky's leader at Tirreno-Adriatico, Richie Porte, abandons the race after suffering from stomach upset
Tour de France champion Chris Froome is back on his bike and training after skipping Tirreno-Adriatico will back problems
Geraint Thomas forced to pull out of Paris-Nice after crashing in final five kilometres of stage seven on Saturday
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