Swiss stocks of uranium held in other countries increased by 40 per cent in 2010 compared with 2009 stores.
Information released by the Federal Energy Office show stores of natural and enriched uranium increased from 124.9 tons in 2009 to 204.9 tons in 2010.
Stocks of reprocessed uranium were reduced by 20 kilograms to 1866.6 tons, while plutonium stock levels remained unchanged.
The materials are being stored by France, Germany, Britain, Sweden and the United States.
The Federal Environment Office said the increase in uranium stocks was the result of the decision to authorise unlimited operation for the Mühleberg nuclear power station near Bern in 2009.
Since 2005, Swiss-owned nuclear stocks stored abroad must be publically declared.
swissinfo.ch and agencies