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.