Noble said they remained on board for two-and-a-half hours while police and military personnel carried out checks, including searches with sniffer dogs.

The passengers were later told they would be taken to an airport in Paris and another plane would fly them on to Rome.

Philip Baum, professor of aviation security at the University of Coventry, said the industry "faces multiple bomb threats every single day" but he had been "surprised" by this week's incidents.

A flight is diverted "only when the threat is credible and the authorities believe there is a potential of explosive device on board", he said.

He said the concern was that "if you issue enough hoax threats, then complacency could set in and a real threat could be ignored - or they could be a diversion".

Marco Chan, an aviation expert at Kingston University, told the BBC the threats were "most likely a hoax, but the system is right to treat it as real".

He said they were being viewed by some experts as "possibly part of a pattern of cheap, deniable disruption" to air travel.

Hoaxes can also come from copycats, he said, but "three days of incidents is too small a sample to separate coordination from contagion".

Two days ago, seven European flights, including two from the UK, were searched after bomb threats were made. No explosives were found on any of the planes after searches.

On Monday, a Wizz Air flight travelling from London Luton Airport to Sofia was escorted by a Hungarian fighter jet to Budapest after a mid-air bomb threat.