The Paris Olympics might have to backtrack on historic plans for an opening ceremony outside a stadium – the first such curtain-raising in the 130-year history of the modern Games – as mounting security concerns force authorities to dust off backup plans for a more traditional event.

Organisers of the tournament, which kicks off next month, aim to create history with an opening ceremony that floats 10,500 athletes on boats for six kilometres down the Seine river in central Paris, but the most fraught security environment in Europe since the 1930s could prevent that.