A formal probe has been launched into possible favoritism regarding a lavish soirée in Las Vegas in honour of French president Emmanuel Macron when he was a government minister, prosecutors announced on Friday.

The judicial inquiry could deal a significant blow to Mr Macron's reformist drive as it implicates his labour minister - at the helm of a landmark labour bill - and her role in organising Mr Macron's high-profile trip to the 2016 trip to the Consumer Electronics Show.

The LA event, which reportedly cost almost €400,000 (£354,000), was orchestrated by Business France, a state body promoting French business interests abroad, at a time when Mr Macron was economy minister under the then Socialist government of his predecessor Francois Hollande.

The then head of Business France, Muriel Pénicaud, went on to be appointed his labour minister.