Pilots fear that 'green' rules demanding that planes burn less fuel could cause planes to crash.

David Reynolds, head of safety at the pilots' union Balpa, strenuously urged regulators to ‘revisit’ some of the environmentally-friendly requirements of aircrafts to prevent a repetition of the Heathrow crash-landing in January.

An eight-month investigation into the dramatic accident on the Beijing to London flight concluded that a rush of ice crystals choked off the fuel supply less than a minute before touchdown.

This caused the engines to lose power and cut out just as the pilot needed extra thrust to land safely.

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Probe: Investigators believe ice in the fuel flow caused BA 038 to crash - but how the ice formed is still a mystery

Mr Reynolds warned: 'These rules need to be looked at again. Fuel flow is an important factor in the safe running of an aircraft engine.

'With reduced burn, that means that less fuel is circulating, which makes it easier for water to separate and turn into ice.'

Referring to the BA crash, he said: 'In this case this was combined with very low temperatures and perhaps fuel which may have had a bit more water than usual - even though it complied with international standards.

"It was an unfortunate combination of circumstances but it does pose questions for all manufacturers, regulators and airlines."

In the wake of January's near-disaster, every long-haul passenger plane in the world now faces strict new safety checks.

They could also be ordered to fly at lower altitudes after investigators admitted last night they have no idea how many other planes may be vulnerable to a 'previously unforeseen threat' of ice blocking the flow of engine fuel.

In a worrying admission, investigators say they still do not know exactly how the ice could have formed in the 777 - because it had not done so before during millions of flight hours.

They now think a unique combination of three events conspired to create the conditions that choked off the fuel. They warn they cannot rule out the possibility of this being replicated in other aircraft. They are:

• The length of time the fuel stayed in the tank at below freezing point in unusually cold weather over Siberia,
• The fact the plane was flying at a steady cruising speed and altitude for a long period which allowed ice to form in the tank because it was using minimal fuel, 
• The sudden demand for a burst of fuel to the engines on landing, which went unanswered as the ice blocked the pipes.
  

Emergency directives were issued last night to 11 airlines - including BA - which operate Boeing 777s. They feature stop-gap measures to prevent a repeat of the conditions which led to the crash on January 17.

This includes a requirement for planes to fly on maximum power mid-flight to prevent the long-term build up of ice in the tanks.

One expert said, that in motoring terms, they have to 'gun the accelerator', even when that is not necessary. Asking pilots to do this inevitably increases fuel consumption.

With costs soaring, this may affect ticket prices.

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However, investigators have also recommended that safety authorities in the U.S. and Europe take far wider action that will affect all planes operating across the globe.

For despite travelling the world in search of answers, the team from The Air Accidents Investigation Branch admit they still don't know exactly how the ice formed.

Tests have only been carried out with Boeing 777s with Rolls-Royce engine and the report said: 'It is unknown whether other aircraft/engine combinations that have already been certificated might also be vulnerable to this previously unforeseen threat.'

They believe the ice formed gradually and was released as the plane prepared to land. Water is naturally present in aviation fuel.

Their report concluded: 'The investigation has shown that the fuel flow to both engines was restricted - most probably due to ice within the fuel feed system.

'This ice is likely to have formed from water that occurred naturally in the fuel whilst the aircraft operated for a long period, with low fuel flows, in an unusually cold environment.'

Significantly, it adds: 'The risk of recurrence needs to be addressed in the short term whilst the investigation continues.'

As well as the action on Boeing 777s, safety authorities are urged to 'take immediate action to consider the implications' for other aircraft.

There are 700 777s in service, of which 220 have Rolls-Royce Trent 800 engines identical to the crash jet. BA currently has 42.

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Miracle: The plane crew somehow managed to land it safely despite both engines inexplicably losing power. Almost all on board emerged entirely unscathed

Current aviation regulations do not take into account the risk of a sudden ice build up within the fuel supply. As a result, authorities on both sides of the Atlantic will review the system for certifying aircraft as airworthy 'to ensure that aircraft and engine fuel systems are tolerant to the potential build up and sudden release of ice'.

The pilots of the 777 - senior first officer John Coward, who was at the controls, and Captain Peter Burkill - were praised as heroes after managing to land safely with 136 passengers and 16 crew.

Flight BA  038 was extensively damaged when it landed short of the runway. One person broke their leg and others received minor injuries.

The investigators dismissed the suggestion that the fuel itself froze or became 'waxy' and suggest use of an anti-freeze additive.

But they point out that the plane flew through unusually cold air over Siberia while travelling from Beijing to Heathrow, although the temperatures of as little as -34C (-29F) were not, they pointed out, unique.

The investigation continues with testing at Rolls Royce in Derby, and Seattle in the U.S., the home of Boeing. The report adds: 'This is the first such event in 6.5million flight hours and places the probability of the failure as being remote.'

The Heathrow incident was the first forced landing of the 777 in the course of 3.9million flights since it entered service in 1995.