French daily Le Monde reported on Friday that the UCI found Froome's and Team Sky's explanation unconvincing and have now referred the matter to its Independent Anti-Doping Tribunal.
Froome had twice the allowable amount of Salbutamol in his urine when tested on 7 September 2017 as he rode to his first Vuelta victory.
The UCI first had Froome's case assessed by its legal anti-doping services with the newspaper reporting it has now advanced to the next step of the process.
If the anti-doping tribunal then rules against the four-time Tour de France champion it will almost certainly land at the Court of Arbitration for Sport (CAS) on appeal.
That lengthy process means Froome remains on track to race at the Giro d'Italia, which starts in Jerusalem 4 May and perhaps the Tour itself in July.
Froome has continued to race since the news broke. He recently finished the Ruta del Sol (10th) and Tirreno-Adriatico (34th) and will fine-tune his Giro training at the Tour des Alpes.
