The British Defense Ministry has admitted to exchanging materials for nuclear weapons with the United States over the past five years.
Nuke materials have been flown between the UK and the US 23 times in the last five years, MoD said without giving details.
The military flights carried tritium, plutonium and enriched uranium, which are needed for UK’s Trident nuclear warheads.
The shipments reportedly started or ended at the RAF base at Brize Norton in Oxfordshire.
The nuclear exchange caused worries among politicians and campaign groups, who are concerned about radioactive contamination.
The ministry, however, insists that the transports were safe.
The Guardian reported last month that two MoD emergency exercises in 2011 and 2012 envisaged such flights might crash. One imagined a leak of enriched uranium and plutonium spreading up to five kilometers across south Wales.
UK MPs later questioned the government, prompting Armed Forces minister Penny Mordaunt to make the announcement.
“All flights were between the UK and the United States on fixed-wing aircraft under the control of UK armed forces.” Details of the cargoes were kept secret “as disclosure would or would be likely to prejudice national security,” she said.
Experts say the UK and the US regularly exchange tritium, plutonium and enriched uranium under an agreement.
Anti-nuclear campaigners have tracked road convoys transporting nuclear materials between the nuclear bomb plants at Aldermaston and Burghfield in Berkshire and RAF Brize Norton.
They say a crash could contaminate large areas of land with potential radiological consequences for people.