French police on Monday detained a couple who were passengers on an Air France flight which was forced to make an emergency landing after a fake bomb was found on board, a police source said.

The couple were taken into custody by border police on their return to France, a day after their flight from Mauritius to Paris was forced to make an emergency landing in Kenya.

Numerous reports in France say one of the arrested pair “has presented himself as a retired policeman”.

He is said to be a 58-year-old man who lives on the island of Reunion. His wife is thought to be the other passenger arrested and she is being questioned as a witness.

France national carrier also said on Monday it had taken legal action after the discovery of the fake bomb.

The airline “filed a legal complaint against unknown persons for endangering the life of others,” a company spokesman said.

A passenger alerted crew members to the item found inside a toilet cubicle on Sunday on board a Boeing 777, which was carrying 459 passengers and 14 crew members from Mauritius to Paris.

(Passengers from the flight that had to make an emergency landing in Kenya. Photo: AFP)

The object, made up of cardboard, paper and a kitchen timer, was found to pose no danger to the aircraft or its passengers, Air France chief executive Frederic Gagey said on Sunday.

He said the “deduction” was that the item had been placed in a toilet cupboard by one of the passengers and said the bomb scare appeared to be the result of a “bad joke”.

France is on high alert after jihadist attacks in Paris in November left 130 people dead, and is one of many countries taking extra security precautions.

Airlines are especially jittery after Islamic State jihadists, who claimed the Paris attacks, also said they were responsible for downing a Russian jet in Egypt in October after smuggling a bomb onto the plane, killing all 224 people on board.

Gagey said there had been three bomb scares on Air France planes in the United States in the past 15 days.