A massive security blunder resulted in three suspected French jihadists walking free in the streets of Marseille after travelling back from Syria.

The incident has embarrassed French authorities and cast doubts over their ability to ensure safety from potentially ill-intentioned home-grown radicals returning to France after receiving training and combat experience in the Middle East.

Imad Djebali, Abdelouahed Baghdali and Gael Maurize were at large on Wednesday, after they landed in the Mediterranean city from Turkey a day earlier - and smoothly passed border controls.

As authorities had been expecting them, lawyers for the three said they were surprised they were not held and were ready to hand themselves to police.

"They came back to France of their own free will. The other reality is that they went through customs in France with their passports without being bothered in any way," said Pierre Dunac, who represents Djebali.

The trio of radicals reportedly travelled to Syria to join Isis, now known as Islamic State, militants in February.

They were known to authorities due to their links to other French extremists and in particular to Mohamed Merah, a 23-year-old who in 2012 went on motorcycle shooting rampage, killing three soldiers, a rabbi and three children at a Jewish school in Toulouse.

Djebali, a childhood friend of Merah, was sentenced to four years in jail for terrorism-related crimes in 2009, while Baghdali was Merah's brother-in-law.

Baghdali's lawyer said the three recently decided to go back to France after growing disillusioned with fighting for IS in Syria.

"It's a classic story of young French or Westerners who dream of Islamic State, a society based on religion, but when they get there they are terrified by the fanaticism, the crimes and torture that is being carried out," lawyer Christian Etelin said.

After their intent to quit jihad was discovered, they were imprisoned by their former fellow-fighters but later managed to escape and cross to Turkey, Etelin claimed.

"They were sure they would be condemned to death and it's for this reason they did everything they could to escape and handed themselves over to the Turkish police to ask for their help and protection," he said.

Ankara contacted French authorities to repatriate them and the trio was initially expected to arrive at Paris Orly airport yesterday.

However they never got there, due to what Defence Minister Jean-Yves Le Drian described as a "huge foul-up" caused by flawed collaboration with Turkish authorities.

According to the Interior Ministry, the three were put on a flight to Marseille after the pilot of the Paris-bound plane refused to let them aboard because of faulty documentation.

The change was not communicated to French police so that the suspect islamist militants were able to walk out of the airport without any trouble.

To add to the government embarrassment, in the afternoon the interior ministry had actually announced the men had been arrested at Orly, and was forced to backtrack a few hours later when the blunder became clear.

Le Drian laid the blame for the incident on Ankara. "Turkey's decision to put them on another flight was particularly misguided," he told France Info radio. "These men will be found very soon".

Nevertheless, the government was accused of "ineptitude" by the opposition.

About 930 French citizens are said to be involved in Islamist movements in Syria and Iraq - with 350 already on site, 170 in transit to the area and 230 attempting to leave.

The numbers have increased 74% in the past eight months, according to government figures.