Britain has restricted the number of sites at which United Parcel Service can screen air cargo at some airports until the courier company improves its security procedures, the government said.

The transport ministry said the restrictions were not based on any immediate or specific terrorism threat and stressed it was not stopping the firm moving cargo through airports.

'It just means they cannot do their own screening, they can use a third party for instance (to carry it out), that will be up to them,' a spokesman said.

'The safety of the travelling public is paramount and our security regime is kept under constant review,' it added in a statement. Contacted at its headquarters in the United States UBS said it could not immediately comment.

The ministry said for security reasons it would not comment further on the details behind the decision.

Last October, the government ordered a security review into all incoming international air cargo after the discovery of a bomb found aboard a UPS plane at a regional airport.

The device hidden in a printer cartridge, and originating in Yemen, was found after a tip-off on a U.S. bound flight at East Midlands Airport in central England.

Prime Minister David Cameron said it was designed to blow an aircaft out of the sky -- possibly over Britain.

Al Qaeda's Yemen-based wing later claimed responsibility for the foiled plot.

A similar device was found on FedEx aircraft in Dubai around the same time. Both bombs were addressed to two Chicago synagogues.-Reuters