DUESSELDORF, Germany (Reuters) - Tension was building up around Team Sky after two days on the Tour de France as rival teams complained that the jerseys they used for the opening time trial had been designed to illegally boost aerodynamics.

Cycling - The 104th Tour de France cycling race - The 14-km (8.7 miles) individual time-trial Stage 1 - Duesseldorf, Germany - July 1, 2017 - Team Sky rider Vasil Kiryienka of Belarus starts the stage. REUTERS/Benoit Tessier

The British team had four riders in the top eight of Saturday’s opening time trial, with Geraint Thomas taking the race leader’s yellow jersey.

Little bubble pads were seen on the upper arms on the jerseys of several Team Sky riders, including that of defending champion Chris Froome.

International Cycling Union (UCI) regulations state: “Garments must not be adapted in any way such that they diverge from their use purely as clothing. The addition of any non-essential element or device to clothing is prohibited.”“It is an actual part of the jersey, it was not added,” race jury president Philippe Marien told Reuters on Sunday.

“We summoned the team’s sports directors to check the jerseys. Nothing was added to them,” said Marien.

Some teams, however, were not happy with the jury’s view. Reuters learnt that at least two rival teams had complained about the Team Sky jerseys ahead of Sunday’s 203.5-km ride to Liege.

“It’s enhanced aerodynamics and the regulations forbid it,” said Frederic Grappe, performance director at the FDJ team.

“According to studies, the estimated gain is about four to seven percent. It’s huge.”

Italian Marco Pinotti, a coach at BMC Racing, told Reuters: “It seems to me this ‘equipment’ is on the borderline with the rules but if it’s judged compliant with the rules then I accept it.”

Asked by a reporter if he had used the jersey before Saturday and what effect it had had on his performance, Thomas replied with sarcasm, saying: “You can borrow the suit if you want, see what time you do.”

Asked again, he said: “I used it at the Giro (in May).”

While some believe Team Sky are bending the rules, their sports director Nicolas Portal said they were working within them.

“There are other teams who have been using it. Other big teams have been using it,” the Frenchman said.

“It’s legal. Everybody knows the rules. It does not give you a big advantage but it’s those little things... so why not use them?

“Everything has been validated. We would not take the risk to cheat and lose everything on the first day. We abide by the rules. Every team needs to work on (within) the rules.”