Over the weekend the Federal Aviation Administration grounded all crop-dusting aircraft after law enforcement authorities found a crop-dusting manual while searching the belongings of a suspected terrorist. Reports also circulated that one suspected hijacker had visited crop-dusting companies in Florida this year.

The discovery heightened fears that terrorists could be or have been planning an assault involving deadly biological or chemical weapons. John Ashcroft, the US attorney general, said the use of such aircraft to spread chemical agents was a potential threat to Americans but noted there was "no clear indication of the time and place of such attacks".

In issuing the flight restriction on Sunday morning, the Federal Bureau of Investigation urged agricultural aerial operators to be wary of "suspicious activity" involving hazardous chemicals. Authorities did not directly link the grounding of the aircraft - about 4,000 across the US - to the September 11 terrorist attacks.

The FAA said it was removing the flight ban on crop dusters at 12:05am on Tuesday. Authorities would not comment on whether the removal meant government concerns had eased over a possible biochemical attack.

Crop dusters would still not be allowed to fly within 20 miles of metropolitan airports nor "do they need to", said an official with the FAA.

Crop-dusting aircraft are usually owned and operated by one pilot. "It's not like you can rent one of these aeroplanes," said John Walberg, the manager of the flight training centre at the University of Minnesota.

"An individual would have to steal the aeroplane somehow and even that would be difficult as most people keep them under lock and key."

The ban has so far not had a significant impact on farmers, some of whom are just finishing spraying their crops in the Midwest. Others in the South are preparing to do so in the next few months.

In Europe crop dusting is less common. In the UK, which has six operators, the Department of Transport said it would review security arrangements for aerial crop spraying. The use of aerial spraying is confined mostly to controlling bracken in the highlands of Scotland and in Wales.

Elsewhere in Europe limited aerial spraying is used in rice fields in Spain, wine-growing areas of Germany's Mosel valley and olive groves on the Greek island of Corfu, but the possibility of a terrorist threat was not considered a high risk.

A spokeswoman for Germany's Federal Ministry of Consumer Protection, Food and Agriculture said: "The aircraft involved are very small and we do not view this as posing any terrorist threat."