Lance Armstrong violated anti-doping rules during a random test in Southern France and could face disciplinary action, the French Anti-Doping Agency (AFLD) said on Thursday.
The seven-times Tour de France champion "did not respect the obligation to stay under (the) direct and permanent observation" of the tester, the AFLD said in a statement.
The 37-year-old American, who returned to the sport in January after 3 1/2 years retirement, denied any wrongdoing on Tuesday.
Armstrong was asked to provide urine, blood and hair samples when returning from a training ride around Beaulieu-sur-Mer last month.
"In a letter sent to the Agency on April 8, UCI (International Cycling Union) president Pat McQuaid said an interpretation of the World Anti-Doping code and UCI anti-doping rules confers the AFLD the right to open a disciplinary procedure against Lance Armstrong," the statement read.
"The AFLD is competent to impose disciplinary sanctions to people who do not hold a French licence but train on the national territory."
Any sanction imposed on Armstrong would be valid on French territory only.
Armstrong is set to ride the Tour de France, which starts in Monaco on July 4.
In a statement on Tuesday, Armstrong explained: "We told the tester we wanted to check with the UCI to confirm who he was and to make sure he wasn't just some French guy with a backpack and some equipment to take my blood and urine.
"Johan (Bruyneel, Astana team manager) stayed with him and in his presence called the UCI to find out what was going on.
"We asked if it was OK for me to run inside and shower while they made their calls and the tester said that was fine."
Armstrong has had a difficult relationship with Tour organisers, the Amaury Sport Organisation (ASO), who said last year the 37-year-old's return to the race would be "embarrassing."
In 2005, French sports daily L'Equipe, owned by ASO's parent company EPA (Editions Philippe Amaury), claimed samples of Armstrong's urine from the 1999 race showed traces of the banned blood-boosting substance erythropoietin.
However, Armstrong never tested positive and was cleared by a Dutch investigator appointed by UCI.
AFLD president Pierre Bordry last year invited Armstrong to have his 1999 samples re-tested but the Texan declined, arguing they could have been compromised.