The Daily Telegraph newspaper said the ministry had also released papers to the Public Record Office describing ways that such a bomb could be smuggled into the country.

A Ministry of Defense spokesman told Reuters he had no comment to make on the report.

Fears over nuclear terrorism have heightened worldwide since the September 11 hijacking attacks on the United States.

Retired engineer Brian Burnell, who worked on the British atomic weapons program, told the paper the plans were enough to enable a terrorist to make an atomic bomb without difficulty.

"These documents should never have been declassified and since the events of September 11 there is a case for removing them from public access," he was quoted as saying.

The main opposition Conservative Party's defense spokesman Bernard Jenkins told the Telegraph the files were "a monstrous free gift to terrorists."

The paper said the files related to the construction of Blue Danube, the first British atomic bomb which was built in the late 1940s and early 1950s.

The plans gave complete cross-sections, precise measurements and full details of materials used for all the components, including the plutonium core and the initiator that sets off the chain reaction causing the blast, the paper said.

Burnell said a prospective bombmaker would need only a basic machine shop and the right components -- including weapons-grade plutonium -- to make the bomb according to the instructions in the files.

The plans, available for anyone to see, are contained in files released to the Public Record Office over the last five years, the Telegraph said.

Last month researchers based at Stanford University in the United States said they had compiled a database of lost, stolen and misplaced nuclear material.

Their research showed that over the past 10 years, at least 88 pounds of weapons-usable plutonium and uranium had been stolen from poorly protected facilities in the former Soviet Union. Most but not all of the material was eventually recovered.

In March, the International Atomic Energy Agency's governing board announced plans to upgrade worldwide protection against nuclear terrorism.

The measures are aimed at helping states assess and eradicate their vulnerability to nuclear terrorism, including the potential use of nuclear material for attacks.