Geraint Thomas will avoid riding close to the crowds lining the route on Friday’s key Tour de France mountain stage after a spectator deliberately grabbed him and nearly caused him to crash on stage 17.

The Welshman, who retained his yellow jersey and a 1min 59sec lead after Thursday’s 18th stage ended in a bunch sprint, said he was staggered after his Team Sky colleague Wout Poels showed him a photograph of “an idiot” almost bringing him down towards the end of the climb up the Col de Portet on Wednesday.

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“Obviously it’s not good and I won’t be riding quite so close to the barriers,” said Thomas, who admitted he did not appreciate how serious the incident was until much later. “I could have quite easily fallen and lost a bit of time. I was going quite fast past him but it certainly moved me off my line. It was a bit of a shock but luckily I was OK.

“But obviously it wasn’t nice. We come here to race our bikes and that’s all we want to do. The whole peloton just wants to do that safely. It’s a bit too much.”

Thomas said he did not believe his incident was specifically related to Team Sky, who have faced an almost daily diet of ire and ill-feeling from some fans during the Tour. “Apparently he was doing the same to Nairo Quintana. I think it was just too much to drink, a bit of an idiot.”

Dave Brailsford, Team Sky’s principal, called the incident “unacceptable” and admitted it was the latest in the long line of issues his team had faced. “I can’t imagine any fair-minded person, regardless of nationality, can watch that and think it has a place in sport. But we’ve been saying that for three weeks now.”

Since the Tour began in Brittany, the four-times winner Chris Froome has been cuffed by one fan, spat at by another, and had an unidentified substance thrown at him, while on Tuesday’s 16th stage riders were accidentally hit with pepper spray after protesting farmers stopped the race.

Froome was also in the wars again on Wednesday when he was brought down by a gendarme as he made his way back to the Team Sky bus after the stage – with footage showing him swearing at the officer, who had mistaken him for a spectator.

Brailsford insisted Froome’s reaction was understandable given the circumstances. “I think if somebody pulls you off the bike unexpectedly, the shock of that, if you think you’re being attacked, every fair-minded person would agree your first reaction to that is going to be emotional,” he said.

Froome played down the incident – which led to his bike being trashed and his bodyguard also coming down – as a misunderstanding. “I was the first rider to come down the descent and one of the gendarmes grabbed my arm as I was passing,” said Froome, who was wearing a grey rain jacket over his race jersey. “Obviously he thought I was a spectator going down the race route. I was going at some speed so I came off obviously, but it was just a misunderstanding.”

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The Lotto manager Richard Plugge told the Guardian that the incident had been caused by organisers changing plans at the last minute to send riders down on ski lifts because spectators had grabbed them instead.

“The big stars of cycling descending in such an environment is not a good idea, after such a mountain stage,” he said. “At least the top 10 should be helicoptered down or in some other way. They gave everything, they pushed 100% but they had to ride down in the cold, through the cars coming up and the other riders.”

Plugge, who is also vice president of the AIGCP, the professional teams association, said he felt sympathy for the gendarmes having to police such a volatile tour. “The world has changed, and there have been some really difficult moments in France, so we understand the tense situation among the policemen,” he added.

The Danish rider Jakob Fuglsang has expressed surprise at the speed of Team Sky’s train during the three weeks – and at Thomas’s brilliant performance. “I’m not surprised the others are faster than me but I’m surprised how much faster they are,” he said. “I just talked with Mikel Landa after the stage and he can’t believe it, either. He says he did less watts last year.”

Fuglsang, who made it clear he was not accusing anybody of wrongdoing, added: “It’s surprising that Thomas had such a great Tour. He was already good in the Dauphiné and here he kept his form perfectly until now – that’s been a surprise and for a guy who’s never finished in the top 10 in a Grand Tour before, holding on to the yellow jersey for so long and defending so well, and seemingly being so strong, that it is a bit of a surprise. But he is a good guy and I hope that he holds on until Paris.”