A terrorist attack on a nuclear facility would be "highly unlikely" to cause instant widespread casualties, a report has said.

But a study released by the Parliamentary Office of Science and Technology (POST) said there could be a long term rise in cancers as a result of any such attack.

POST, an office of the two houses of parliament which provides analysis of public policy issues related to science and technology, also said that nuclear plans were not designed to withstand attacks from large aircraft.

The report added that existing safety systems would offer protection against any release of radioactive materials.

"A successful attack would be highly unlikely to cause large numbers of instant fatalities," said the POST.

"Although it would have the potential to affect extensive areas of land and cause large numbers of long-term cancers, its impact would depend on how effectively appropriate contingency plans were implemented.

"Even an unsuccessful attack could have economic and social repercussions and affect public confidence in nuclear activities such as power generation."

Commercial nuclear power gives rise to most of the UK's total radioactive inventory, of which the largest amounts are at the Sellafield reprocessing plant in Cumbria and at Dounreay in Scotland, said the report.

There are also 13 generating power plants, six decommissioning power plants and various other military and civilian sites across the country.

The core of a nuclear reactor in a power plant contains over 100 tonnes of radioactive material at several hundred degrees Celsius.

"Its safety therefore relies on controlling the nuclear chain reaction, cooling the reactor core and containing the radioactive material," noted the study.

Terrorists might attempt to cause a release either by breaching the reactor shielding with "a violent impact or explosion" or indirectly by damaging enough critical safety systems.

However studies carried out for the Sizewell B public inquiry concluded that if a military aircraft were to strike the reactor building there would be a three to four per cent chance of uncontrolled release of radioactive material.

And deactivating safety systems appears unlikely given the "strength in depth" engineering and the high degree of access that would be required.