Forrest, the 30-year-old maths teacher who ran off with a 15-year-old pupil in September last year, pleaded not guilty to abduction in a British court in January.
But his lawyers are also pursuing his case in France, arguing that he has faced different charges in the UK to those that figured on the European arrest warrant that led his transfer from France.
On 10 October last year a Bordeaux court ordered Forrest to be handed over to the British due to a warrant for abducting a child, which can mean a maximum of seven years in prison.
But on 15 January, when Forrest was already in a British prison awaiting trial, it authorised them to prosecute him for sex charges that could mean a 14-year sentence, ignoring the terms of the warrant, they argue.
The lawyers have now been given permission to take the case to highest court of appeal, the constitutional council, sources told the AFP news agency Friday, judging the complaint to have a “serious character”.