Hope of finding remains from the downed Air France Flight 447 Airbus were fading tonight as investigators confirmed that faulty instrument readings had played a role in the crash in the Atlantic which apparently killed all 228 aboard.

French Ministers voiced frustration after the Brazilian forces said they had been mistaken in identifying flotsam collected from the ocean as coming from the Airbus A330 that disappeared in a storm on Monday on a flight from Rio to Paris.

They also said that they had mis-identified a fuel slick on the surface.

"Time is against us," said Dominique Bussereau, the Transport Minister. "We must do everything we can to find the flight recorders and certainly enlarge the search zone," he said.

The Brazilian admission meant that there is no longer any certainty about the area, roughly 800 miles off north-eastern Brazil, in which flight AF447 is believed to have come down.

No French aircraft had spotted any identifiable debris since they arrived on the scene late on Monday.

"We have been saying for several days that we have to be extremely cautious," Mr Bussereau said. "Our planes and ships have seen nothing."

To find the "black box" recorders, which lie about 12,000 feet under the surface, search vessels must lower sonar receivers deep into the ocean. To have any chance of receiving the locator 'pings', they must have an approximate position where the recorders lie.

In the absence of the detailed data from the recorders, the investigators are relying on a cascade of alert messages which were transmitted automatically by the airliner for four minutes to Air France's maintenance base in the closing minutes of the flight.

The Bureau of Accidents and Analysis (BEA) confirmed today that the aircraft's instruments had registered conflicting speed readings. They offered no analysis, but this confirmed leaks from Air France that abnormal air-speed indications were part of the breakdown, which began with the automatic pilot and computer flight control system being disconnected.

At the same time, Airbus put out a circular to operators of its aircraft, reminding them of the correct procedures to follow when airspeed readings - one of the most vital parameters for a pilot - become faulty. Crews should maintain power and pitch and, if necessary, level off the plane and start troubleshooting procedures as detailed in operating manuals, said an Airbus spokesman.

Theories on the breakdown focused on the possibility that ice, which can be dense in a violent tropical storm, could have blocked the pitot tubes, the outside probes which read the pressure of oncoming air.

Even with full instruments, an airliner at high altitude risks losing control in the extreme turbulence inside a storm. At 35,000 feet and almost fully loaded, the Airbus would have had a narrow margin between the low speed at which it would stall and the higher "overspeed" at which violent buffeting near the sound barrier would endanger the flight. This narrow margin is nicknamed "coffin corner" and experienced Airbus pilots said today that hand flying a jet in the predicament of the stricken Airbus would have been near impossible.

It was also unclear how other breakdowns in the control systems could have been related to the airspeed failure. The automatic pilot and electronic flight system disconnected at the same time. The final alerts reported cabin losing pressure and excess vertical speed, meaning the jet was plunging towards the ocean.

The latest Airbus crash has renewed suspicion in the flying world over the highly automated systems of European aircraft family. For years, some pilots have said that they do not like the computers having so much control over their aircraft.

Computer anomalies have been blamed for a series of incidents, such as the upset of a Qantas A330 in flight over the Indian Ocean last October.

Jean-Pierre Albran, a captain of Boeing 747 jumbo jets, told le Parisien newspaper today that the Airbus family of airliners was "a little worrying" for old-fashioned aviators.

"You do not always understand the aircraft. It is designed by engineers for engineers and not always for the pilots."

As relatives of the victims waited in Brazil to be flown to visit the crash site, Air France today changed the number of its Rio-Paris flight to AF 445. A new number is an airline tradition after a flight number has come to be identified with disaster.

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