They admitted that they did not know who was behind the bomb, which was discovered at Ancona, on the Adriatic coast.

Andrea Tarroni, head of the Italian pilots’ union, said that the circumvention of airport security measures was “extremely serious and worrying”.

Suspects include local drug traffickers, or even disgruntled airport or airline employees, as well as Islamic terrorists and home-grown extremists such as the recently revived Red Brigades.

The Adriatic resorts, although largely devoted to family beach holidays, have been infiltrated by East European and other foreign gangs who use the resorts’ less respectable nightclubs as a front for drugs and prostitution.

Police said the device did not bear the hallmarks of “professional” terrorists. It consisted of explosives in packages “like cigarette packets” from which wires were protruding. The packets were bound together with adhesive tape and stuck beneath a seat at the rear of the aircraft.

The French-Italian ATR 82 turboprop started its journey in Nice and arrived in Ancona after stopovers in Florence and Rome. The bomb was found after an anonymous call to Ancona police. It was removed safely and exploded by bomb disposal experts, causing “a sizeable blast”.

The incident occurred as Giuseppe Pisanu, Italy’s Interior Minister, said in a speech in Venice that the country faced threats from organised crime, extremist Muslim groups and from the Red Brigades, who have murdered two government advisers.

Signor Pisanu said although most Muslims in Italy lived peaceably alongside their Italian neighbours, imams at mosques in Rome and Milan had made “inflammatory” speeches inciting Muslims to use violence.

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