Alberto Contador's bike among those checked for hidden motors at Giro d'Italia (video)

Additional reporting by Gregor Brown

Five riders had their bikes checked for hidden electric motors after stage 18 of the 2015 Giro d'Italia on Thursday, including that of race leader Alberto Contador (Tinkoff-Saxo).

Stage winner Philippe Gilbert (BMC), Ryder Hesjedal (Cannondale-Garmin), Rinaldo Nocentini (Ag2r) and Kenny Elisonde (FDJ) also all had their bikes checked.

"Unannounced bike checks to clarify the absence of hidden motors were held," read a commissaire's statement issued after the Giro stage on Thursday evening. "We proceeded to verify the bicycle by removing the saddle, or the pedal axle to view the inside of the bottom bracket".

Contador and Hesjedal both rode away from their general classification rivals on stage 18's key climb of Monte Ologno, with the pair gaining over a minute on the likes of Mikel Landa, Fabio Aru (both Astana) and Leopold Konig (Sky).

Union Cycliste Internationale (UCI) commissaires have been screening bikes for hidden motors in several races this season, although to date there has been no report of finding anything suspicious.

For example, 37 team bikes from Trek Factory Racing, Etixx-QuickStep, Orica-GreenEdge, Katusha and Giant-Alpecin were dismantled after the 2015 Milan-San Remo in the search for hidden motors.

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Allegations and rumours of the use of electric motors in bikes used by professional riders first became well known in 2010, after Fabian Cancellara won the Tour of Flanders and Paris-Roubaix

The rumours were given added weight when former pro Davide Cassani appeared on Italian television with a motorised bike, claiming that similar machines had been used by professionals 'since 2004'.

The subject hit the headlines again last year, when a widely-shared video on the internet purported to show Hesjedal's bike moving on its own after crashing during the 2014 Vuelta a Espana. Hesjedal was one of those whose bike was checked at the Giro.

https://www.youtube.com/watch?v=8Nd13ARuvVE

https://www.youtube.com/watch?v=ideiS-6gBAc

The subject of 'motorised doping' was also brought up in the Cycling Independent Reform Commission (CIRC) report into professional cycling published in March, which included the line: “The Commission was told of varying efforts to cheat the technical rules, including using motors in frames. This particular issue was taken seriously, especially by top riders, and was not dismissed as being isolated."