The Office for Nuclear Regulation (ONR) has published figures for the UK's annual holdings of fissile materials for the year 2013. The figures have been reported to the International Atomic Energy Agency (IAEA).
At 31 December 2013 the UK held a total of 123.2 tonnes of civil unirradiated plutonium, up from 120.0 tonnes the previous year. The majority of this (118.8 tonnes) was in the form of unirradiated separated plutonium in product stores at reprocessing plants. A further 31 tonnes of plutonium were held in the form of spent civil reactor fuel. The UK's civil plutonium stocks have shown a gradual long term year-on-year increase over recent years.
The UK's stocks of civil high enriched uranium (HEU) also increased slightly from 1,396 kilogrammes in 2012 to 1,398 kilogrammes in 2013.
Military stockpiles of fissile material are held separately from civil stocks and are not reported by the government.