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About half of the 500 or so British jihadist rebels in Syria and Iraq are Londoners, Scotland Yard chief Sir Bernard Hogan-Howe revealed today.

As news emerged of two female Italian aid workers being kidnapped by Islamist militants, Sir Bernard warned of the possible threat from hundreds of British fighters returning home if the Islamic Front was defeated or suffered a reversal of fortune.

He said police were preparing contingency plans to deal with fighters returning to the UK: “There are a significant number of people who are at risk of having been radicalised, militarised and densensitised to violence who might want to come home.”

Sir Bernard said police could investigate them for committing offences abroad and question people on their return.

The Italian women who have been kidnapped ignored their parents’ objections to sneak into Syria. Greta Ramelli, 20, and Vanessa Marzullo, 21, were working on humanitarian projects in Aleppo when they were snatched.

Although details of the kidnapping remain unclear, it is believed they were taken by an Islamist group which has previously kidnapped Western activists and journalists, and have now been transported to the home of the head of the local “Revolutionary Council”.

The Italian foreign ministry said the country is working “belly to the ground” to find them but that contact with the women is “impossible”.

Ms Marzullo’s father Salvatore Marzullo today told of his “sickening” fear for his daughter’s safety.

He told Italian newspaper Corriere Della Sera he had pleaded with his daughter not to travel to Syria to distribute aid, but she had insisted on doing so. He said: “What could I do? Vanessa is an adult, a golden girl, brave and responsible. I tried to reason with her, I tried to convince her in every way I could not to do what she had in mind.

“I could not stop her from doing what she wanted. It’s not like you can take a person and tie them down.”

He said his daughter’s interest in activism grew while studying linguistic and cultural mediation at the University of Milan, and that she had great empathy for the plight of Syrians.

According to their Facebook pages, the two women have made several visits to Syria since 2012.

Italian journalist Daniele Raineri of Il Foglio is believed to have been seized with the women but escaped.