In a speech on Monday, Macron said for the first time that instead of the teams sailing down the Seine on barges, the ceremony could be "limited to the Trocadero" building across the river from the Eiffel Tower or "even moved to the Stade de France".

The Paris organisers have devised a ceremony that is unprecedented in Olympic history, breaking with the tradition of opening the Games in the main stadium. Under current plans, up to 10,000 athletes will sail along a six-kilometre stretch of the Seine in some 160 barges before gathering at the Trocadero for a ceremony.

With war currently raging in Ukraine and Gaza, concerns have been raised that the ceremony could leave athletes potentially vulnerable to attack. "I'm obviously concerned," Anson told Sky News in an interview broadcast on Wednesday to mark 100 days until the opening ceremony on 26 July.

"It isone of the most important things we have to manage from a risk perspective. "We'd be naive if we didn't think about it. Our number one priority is the safety of all our athletes, our wider entourage and our fans."

Macron said the opening ceremony would take place under high security, but that the organisers had contingency plans in place. "This opening ceremony... is a world first. We can do it and we will do it," Macron said in an interview with BFMTV and RMC.

"We have set up a security cordon, which is going to be very big, where we are going to check all the people coming in and going out". But, he added, "there are plan Bs and plan Cs, we are preparing them in parallel... We will analyse this in real time".

The contingency plans include moving the ceremony to the Stade de France in the north of Paris, the main venue for athletics - the centrepiece of the Olympics.  Anson added that the opening ceremony "is a high-risk environment and it's got to be managed accordingly, but the French are very, very aware of that".

Holding the Olympic opening ceremony on the Seine is the "main plan" and "very likely", Tony Estanguet, president of the organising committee, said on Tuesday, the day after Emmanuel Macron's comments suggesting alternative venues at the Trocadéro and the Stade de France.

Echoing their British counterparts, US Olympic chiefs said they were "not surprised" by French President Emmanuel Macron's comments that the opening ceremony of the Paris Olympics could be moved from the Seine to the national stadium in the event of a terrorist threat.

Speaking at a media event for the US Olympic team in New York, United States Olympic and Paralympic Committee (USOPC) chief executive Sarah Hirshland said she expected Paris 2024 officials to have contingency plans in place.