Fragments of metal torn from the wheels after two tyres burst on take-off are now believed to have caused Tuesday's Concorde disaster in which 114 people died.

Fragments of metal torn from the wheels after two tyres burst on take-off are now believed to have caused Tuesday's Concorde disaster in which 114 people died.

The French accident investigation board yesterday ruled out the possibility that the fire on the left side of the aircraft had started in the engines, as first thought. The precise cause of the blaze is still being investigated but it is believed that splinters of metal from the wheels may have severed fuel lines underneath the left wing.

The aircraft crashed a minute after take-off because of the failure of the left-side inner engine and the partial failure of the engine alongside it.

The precise cause of the engine failures is unclear. They may have been starved of fuel or damaged by shards of metal or rubber from the broken wheels.

But the accident investigation board said yesterday that "no debris from within the engines" had been identified along the brief flight path of the Concorde, until the crash site itself. This suggests fuel starvation rather than engine failure.

These official but provisional findings imply that the accident was caused by structural weaknesses in the aircraft, not by human error. As a result, the remaining Air France fleet of five Concordes was grounded until further notice by the French government yesterday, increasing pressure on British Airways to do the same.

The fact that the 30-year-old aircraft could be so catastrophically vulnerable to something so simple as a burst tyre caused French aircrew unions to call for its indefinite grounding.

The French Transport Minister, Jean-Claude Gayssot, called for the British and French authorities and airlines to get together to work out "new safety measures and safety checks".

The new evidence on the causes of Tuesday's disaster comes from the examination of the fragments of tyre and other debris found on runway 26 at Charles de Gaulle airport. It also comes from a preliminary examination of parts of the wreckage recovered from the crash site.

More than 100,000 fragments of the aircraft, some of them no bigger than one centimetre square, have been gathered in a hangar at a French air force base three miles away.

The initial examination of the two black boxes from the crashed aircraft gave investigators a wealth of confusing information on a series of faults that afflicted Concorde number F-BTSC on Tuesday. One engine caught fire and closed down; another lost power. The landing gear failed to lift on take-off; the debris of tyres and other parts of the wheels were found on the runway.

The new evidence has clarified the likely chain of events that destroyed flight AF4590. Accident investigators, including experts from Britain and the US, believe one, or more likely, two tyres on the left side under-carriage burst soon after the plane began its take-off run.

A fire then broke out under the left-side wing. This fire "probably originated outside the engines," the BureauEnquête-Accidents said yesterday. Officially, the cause has yet to be determined. Aviation experts in Paris say the most likely cause is that the landing gear, with two wheels tyres deflated and two still intact, scraped the runway as the plane gathered speed, spraying out splinters of metal that damaged the fuel lines in the wings.

One picture taken by a tourist of the aircraft taking off appears to show that the original fire was in the left-side wing, rather than the engines.

There have been several incidents of tyres bursting on Concordes on take-off before, but without serious consequences. Aviation experts said that the aircraft has a short, very fast take-off run, which places unusual pressure on the wheels.

The two black boxes - the cockpit voice recorder and the instrument recorder - show that the aircraft was on the point of lift-off before the fire was noticed by air traffic controllers. The Concorde lurched into the air with one engine out of action. Less than a minute later, it veered to the left and crashed, when the other left-side engine suffered a partial loss of power. One crucial question is whether air traffic controllers should have seen the fire earlier, in time to prevent the plane from taking off.