Britain has stockpiled 100 tonnes of plutonium - enough to make 17,000 nuclear bombs, according to a report by the Royal Society.
The group, an independent science academy, warns that the material could be used to make "a crude nuclear bomb", and is calling for its disposal.
Report chairman Professor Geoffrey Boulton said: "The stockpile has grown whilst international nuclear proliferation and terrorist threats have increased.
"Just over 6kg of plutonium was used in the bomb which devastated Nagasaki and the UK has many thousands of times that amount.
"We must take measures to ensure that this very dangerous material does not fall into the wrong hands."
Professor Boulton added that the best option would be to convert the plutonium into pellets "which won't disperse in the atmosphere so readily". The pellets could be used to power nuclear reactors,
The Republic's Fine Gael party criticised the British government and said it is incredibly worrying for Irish safety and security.
The party's Foreign Affairs spokesman, Billy Timmins TD said: "It is alarming that, when the risks from terrorist threats have never been more dire, our closest neighbour is hoarding so much of this lethal substance.
"The 100 tonnes of plutonium stockpiled in the UK mainly comes from spent uranium fuel in their nuclear power plants.
"Danger lies in the possibility of just a miniscule amount being used in a crude nuclear device or ‘dirty bomb’.
“In that catastrophic event, the danger to Ireland would be incalculable. I have been less than reassured with statements from the British Government.
"I'm not asking the UK authorities to reveal sensitive national defence information but a more complete reassurance is needed in light of today's information.
"I will be raising the issue with the Irish Department of Foreign Affairs and asking them to intervene with their British counterparts immediately and urging intervention at an EU level."