A spokesman for the Department of Trade and Industry said no-fly zones had been imposed around 11 nuclear plants. Seven installations were already covered by long-standing bans.

The restrictions prevent aircraft from flying within two miles (3.2 km) of a nuclear plant or below 2,000 feet (600 metres), the spokesman said.

"These overflights were causing concern," the spokesman said. "This will help prevent air accidents."

Since the September 11 attacks on New York and Washington, terrorism experts have drawn attention to the vulnerability of nuclear plants to suicide attacks using airliners.

Two Royal Air Force Tornado warplanes were scrambled over the Sellafield nuclear reprocessing plant in northwest England late last month after a security alert which proved to be a false alarm.

France has installed surface-to-air missiles around a nculear reprocessing plant at Cap la Hague and said it will use warplanes to shoot down any hijacked aircraft threatening nuclear installations.

British Defence Secretary Geoff Hoon indicated last week that the country's part time soldiers could be used to guard potential targets.

He said there was "no reason" why they should not be used to guard nuclear power stations.

Last month, the U.S. nuclear power station at Three Mile Island in Pennsylvania was briefly put on high alert, and nearby airports were closed, after a security threat.