The Civil Aviation Authority expects to hear from its United States counterpart, the Federal Aviation Authority, "early this week" about whether the changes are needed.
A spokesman for the CAA conceded it was "more likely than not" that the FAA would adopt the recommendations of the US's National Transportation Safety Board - published last Friday - as a directive. It is almost certain that the British organisation will immediately pass on the effect of the directive to carriers based here, such as British Airways and Virgin Atlantic.
The cost of any design changes, which could run to millions of pounds world-wide, will probably be born by Boeing, which might also have to pay for disruption caused by grounding the aircraft.
According to the CAA, which insisted that jumbo jets were safe, the directive is expected to highlight two potential areas of concern. It will require extra insulation between the central fuel tank and the air-conditioner unit to stop fuel vapours over-heating, and demand the tank maintains some fuel reserves to prevent vapour build-up.
Both changes address the theory of the NTSB - the US equivalent of the Air Accident Investigation Branch - that an explosion from within a fuel tank was a likely cause of the TWA disaster, which killed 230 people. Last Friday the board, which found no sign of a bomb or missile, wrote to the FAA outlining a list of urgent modifications of the tanks.
Yesterday MPs called for urgent action over the recommendations. Gwyneth Dunwoody, Labour member for Crewe and Nantwich, said: "The volatility of aviation fuel in empty tanks has been a worry to the aviation industry for some time. Responsible airlines will certainly act but the Department of Transport has a responsibility to check on behalf of British passengers."
However, there was a growing fear among some experts that the recommendations may reflect a political desire to be seen to be doing something. David Learmount of Flight International said: "The NTSB do not know what the cause is ... I believe their thinking is that because they do not know the cause, they cannot use this as an excuse for doing nothing."
Yesterday, British Airways, which has around 60 747s, said it had been carrying out its own checks since September and had found no problems. The airline said it would implement any CAA directive immediately, as did Virgin Atlantic.
US sky giants
The Boeing and McDonnell Douglas aircraft manufacturers said yesterday that they planned to merge to create the world's largest aerospace company, with sales of $48bn (pounds 29bn), some 200,000 employees and an order book of civil and military aircraft worth $100bn.
The new group, which will retain the Boeing name, will manufacture about three-quarters of the world's commercial airliners and pose an increasing threat to Airbus Industrie, the European consortium, which has a 20 per cent share.
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