Najar's team-mate and fellow Argentinian Gaston Javier also failed a drugs test at the event as he tested positive for anabolic androgenic steroids.

The SEP de San Juan team are now set to be suspended by the UCI as two of their riders have fallen foul of the anti-doping rules in the past 12 months.

Both riders have been provisionally banned.

Najar's sample, taken on January 21, revealed the presence of continuous erythropoiesis receptoractivator (CERA), a blood booster similar to erythropoietin.

The 24-year-old, whose triumph at the Vuelta a San Juan was considered a surprise, was the subject of intense speculation regarding a possible positive test. 

According to Cycling Weekly, organisers of the event tweeted that a doping positive would be revealed soon after the conclusion of race before it was swiftly deleted.

Javier tested positive in a sample collected on January 23.

In a statement, the UCI said the tests were part of "intelligence-led doping controls" and confirmed SEP de San Juan, who compete on the UCI Continental circuit, could be suspended.

"In accordance with UCI anti-doping rules, the riders have been provisionally suspended until the adjudication of the affair," the statement read.

"These two cases within the same 12-month period trigger the application of article 7.12.1 of the UCI anti-doping rules providing for the suspension of the team from 15 days to 45 days.

"The UCI will refer the matter to the Disciplinary Commission which will render a decision against the team in due course."

Ignacio Maldonado of Uruguay also failed a drugs test during the race.