Michael Matthews is out of the Tour de France because of a health condition which, if left untreated, could have killed him.

The Australian cycling star was at a Jayco AlUla altitude training camp when team doctors discovered signs of a pulmonary embolism.

Matthews immediately stopped all physical activity until further notice and is off the team’s roster for the July Tour.

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A pulmonary embolism is a blood clot that goes to the lungs and blocks an artery, restricting blood flow. If untreated, it can be life-threatening.

Matthews would have been a key rider for the Australian team at the Tour. He has won four stages at the race and also took out the green points classification in 2017.

He had his first win of the year early last month at the Eschborn-Frankfurt one-day race in Germany.

Matthews made his Olympic debut last year at the Paris Games, competing in the road race.

The team said in a statement that Matthews’ health is stable.

“The medical team are now thoroughly investigating the extent of the issue and possible cause, to define a safe and optimal recovery process for the athlete,” the statement read.

“During this period of investigation, as a safeguarding measure, Matthews will refrain from competition to ensure there is no risk to his health and well-being, therefore ruling out his participation in the upcoming Tour de France.”

Matthews posted a message to fans on his Instagram account on Saturday.

“Thank you everyone for the kind messages, I will get around to replying to you all, just taking a few days to process the news but my condition is OK and I will make a full recovery, it will just take some time. Thank you all,” he wrote.