Airline passengers will be able to carry any amount of liquid in hand luggage under Government plans to relax restrictions after the upgrade of airport X-ray machines.

Passengers will no longer be limited to 100 millilitre bottles and will be able to buy duty-free alcohol without the risk of it being confiscated.

The screening technology is undergoing secret tests at laboratories in Germany, where scientists are checking its accuracy in detecting bottles of hydrogen peroxide and other liquids that can be turned into bombs.

The ban on liquids was imposed overnight in August 2006 after the discovery of an alleged British terrorist plot to blow up transatlantic flights. Eight men are on trial at Woolwich Crown Court accused of planning to detonate suicide bombs on seven flights departing from Heathrow within hours of each other.

The liquid ban has cost Britain's aviation industry more than £100million in increased security costs and lost duty-free sales. BAA, which has seven British airports including Heathrow, Gatwick and Stansted, had to employ 3,500 more screening staff. Two tonnes of alcohol a month are still being confiscated at Heathrow alone, along with thousands of bottles of perfume and other toiletries.

A senior Whitehall source said that the ban would be lifted at a limited number of airports, possibly as soon as this autumn.

“The restrictions will start to be removed in six months to a year's time and passengers will be allowed to carry any size of bottle they like inside their hand luggage, just as they were before the summer of 2006,” the source said.

“Airports will have to demonstrate that they have invested in the technology, so it will not be a blanket lifting of the restrictions.” He said that a BAA airport would be involved in the trial because the company had already installed advanced threat identification X-ray machines.

These machines, costing more than £100,000 each, have the capacity, after a software upgrade, to distinguish between harmless liquids and potentially explosive ones.

Philip Baum, the editor of Aviation Security magazine, said that a selective lifting of the liquids ban would add to the widespread confusion among passengers about different security requirements in different countries.

“We need much greater harmonisation of security procedures and any changes will have to be communicated very carefully to passengers. People may hear the ban has been lifted and wrongly assume it applies to their airport.”

Thousands of passengers a day misinterpret the ban. Manchester airport has listed the variety of ways passengers had tried to circumvent it. One man with bottles of frozen water claimed that they were solids and therefore exempt; they were confiscated. Another passenger drank a bottle of vodka in front of security staff to avoid having it confiscated; he was later removed from the flight for being drunk.

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