Increasingly damning evidence suggested France was aware Mr Mehar posed a threat.

French intelligence had previously alerted security services in Spain that Mr Merah was planning to travel to the Costa Brava to attend a meeting of Islamist activists, suggesting they considered him dangerous.

The warning said that he was probably on his way to attend a suspected Salafist congress.

The prosecutor said that he had been arrested by Afghan police in 2010 in Kandahar and handed over to US army troops, who put him on a flight back to France – a claim US military did not confirm on Thursday.