Riots could break out in cities across France after results are announced of the first round of the presidential election, intelligence services have warned, as the most unpredictable vote in decades goes ahead amid a heightened terror alert.

Trouble is almost certain if the far-Right leader Marine Le Pen and her far-Left counterpart Jean-Luc-Mélenchon are the two candidates who make it through to the second and final round on May 7, according to a report issued by French intelligence services.

The confidential document, leaked to Le Parisien newspaper, said that at the top of the list of potential security problems as millions of France cast their ballots was the “jihadist threat.”