Laruns, France (AFP) – Dutchman Tom Dumoulin lashed out at Tour de France rival Primoz Roglic on Friday, claiming the Slovenian's stage 19 victory came thanks to some timely pacing by a motorbike on the race.

"I am super disappointed," said Dumoulin, who finished the stage in sixth place at 19secs behind Roglic to see his deficit on yellow jersey holder Geraint Thomas grow by six seconds to 2min 05sec.

"It doesn't make sense. That bike should have been behind us.

"Primoz sat on his (top) tube and powered away from us on a flat section. I was sprinting full gas, and still couldn't catch him."

After cresting the summit of the Col d'Aubisque just behind Poland's Rafal Majka, Roglic pulled away from a small group of frontrunners in a bid to take time from defending champion Chris Froome, who he is fighting for a podium place in Paris on Sunday, on the 20 km descent to the finish.

Despite leading by just a handful of seconds, Roglic's lead stretched after he had been passed by the motorbike. Photographers, cameramen and race officials all use motorbikes during the race.

In the end, it proved decisive as Thomas, Dumoulin and the rest of the group were unable to catch the Lotto-Jumbo team rider.

"The way he did get the gap was a little surprising," said Thomas, whose sprint for second place, and the six bonus seconds on offer, helped him increase his advantage over Dumoulin marginally ahead of the penultimate stage time trial on Saturday.

"I was concentrating on the corners, following Tom's wheel. There has been an issue with motorbikes getting too close, but I didn't see it."

Dumoulin, the defending world time trial champion and last year's Giro d'Italia winner, praised an otherwise impressive performance by Roglic.

He suggested the Slovenian had simply benefitted from circumstances.

"Roglic was better, that is not up for discussion," he said.

"He couldn't do anything about it, he was the strongest and claimed a nice victory."

Using the slipstream of motorbikes in bike racing is illegal, although it is not an unknown phenomenon.

Holding on to car, however, is harshly punished.

In 2015, former Tour de France champion Vincenzo Nibali was thrown off the Tour of Spain after holding onto a team car while chasing back on following a crash in the final 30 kilometres of stage two.

When asked his version of events, Roglic refused to confirm or deny the allegations.

"What can I say? I can't comment on things on which I don't have influence over," he said.