European Union politicians have paved the way for French far-right leader Marine Le Pen to lose her parliamentary immunity over the tweeting of pictures of ISIL atrocities, according to officials from the 28-nation bloc.

French prosecutors opened an investigation in December 2015 into the graphic series of tweets by the National Front (FN) leader, which included a photo of the decapitated body of US reporter James Foley.

A European parliament official said on Tuesday that 18 lawmakers from the Legal Affairs Committee voted in favour of removing the French presidential candidate's immunity and three voted against.

The EU parliament as a whole must now vote - reportedly on Thursday - on the issue, but committee decisions are generally followed by the assembly.

The lifting of the immunity would permit legal action against Le Pen, who is a member of the European parliament.

The offence being considered is "publishing violent images", which under certain circumstances can carry a penalty of three years in prison and a fine of $79,650.

EU officials said that the vote came after a request from the French judiciary.

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Responding to the vote, Le Pen said: "This only shows French citizens what the EU is, what the European parliament is and that it's all part of the system that wants to stop the French people's candidate that I am".

Florian Philippot, a vice president of FN, defended the Eurosceptic candidate's Twitter posts.

"Showing and naming the horror of Islamism allow us to fight against it," Philippot told the Reuters news agency.

Le Pen has previously refused to attend a French police interview over the investigation, citing her status as an MEP.

But her head of cabinet has been placed under formal investigation for "the dissemination of violent images".

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The French leader, locked in an increasingly tight three-way race to succeed Francois Hollande this spring, has already seen her earnings as MEP cut for a different case involving alleged misuse of EU funds.

She has denounced the legal proceedings against her as political interference in the campaign, where she is the leading candidate. She has called for a moratorium on judicial investigations until the election period is over.

Le Pen is expected to win the first of the two election rounds but likely to lose in a runoff, according to polls, which also show that her legal battles seem to have little effect on her supporters.

Le Pen's immunity has been lifted before, in 2013. She was prosecuted in 2015 for "incitement to discrimination over people's religious beliefs", for comparing Muslims praying in public to the Nazi occupation of France during World War II.

Prosecutors eventually recommended the charges be dropped.