Hope of finding the flight recorders of Air France Flight 447 were dashed today after hopes were raised by the detection of faint sounds from the seabed.

French vessels searching for the black boxes off Brazil picked up sounds that could have come from the black boxes of the Airbus A330 that crashed in the Atlantic on June 1.

Technicians on the vessels, which include a nuclear submarine and ships using US listening gear, later concluded that they did not come from the sonar transmitters attached to the flight recorders.

According to Le Monde newspaper, the Nautile, a French mini-submarine, was launched to inspect the ocean-bed before the sounds were declared a false alarm.

The searchers have been combing an area with a radius of 50 miles in the hope of detecting the pinging sound from the boxes that can be heard only up to about two miles.

Since the battery life on the recorders is no more than 30 days, only a week remains if there is a chance of finding the orange-painted boxes. They are likely to be lying in very mountainous terrain some 12,000ft (3,650m) below the surface.

Even if the data recorders are located, their recovery would prove very hard. The Nautile, which was the first vessel to retrieve relics from the Titanic in the mid 1980s, uses a robot to manipulate objects in the ocean. If the boxes are still in position inside the wreckage of the Airbus tail, it may not be able to extract them.

The aircraft broke up at altitude as it flew through a storm about 600 miles off northeast Brazil on a flight from Rio de Janeiro to Paris. All 228 aboard are presumed dead although only 50 bodies have been recovered from the crash zone, along with hundreds of pieces of debris and baggage.

Without data from the black boxes, the investigators may still be able to conclude the probable causes of the crash, because a satellite transmitter on board the aircraft automatically sent a series of data messages to Air France's Paris base during the final four minutes of the flight.

These reported a cascade of breakdowns that began with faulty speed readings and then a abnormal behaviour by the flight computers.

Suspicion has fallen on the computers and they way in which they interpreted the "incoherent" speed data. The origin of the faulty readings is thought to have been the obstruction of the three external probes, known as pitot tubes, possibly by ice.

Yesterday officials said that they had identified 11 of the 50 bodies so far recovered from the crash site, by using dental records and fingerprints. Of those identified ten were Brazilian, although no names have been released.

The passengers on board Flight 447 came from 32 countries, including France, Brazil and Germany. There were five Britons.

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