Exports of ivory antiques from Britain to Hong Kong, a major centre for illegal wildlife trading, more than doubled last year.
Conservation groups said the figures showed that Britain was contributing to the crisis threatening African elephants by helping to sustain a trade that masked sales of poached ivory.
The government promised last week to ban the sale of all ivory artefacts less than 70 years old.
The wildlife charity Tusk and the Zoological Society of London have called for the ban to be extended to much older items. They argue that modern ivory is often passed off as antique and that poachers need to be sent an unambiguous message that ivory has no commercial value.
A total of 2,524 ivory pieces were sent from Britain to Hong Kong last year, up from 1,141 the year before, according to figures obtained by the campaign group WildAid from Hong Kong’s Agriculture, Fisheries and Conservation Department.
Britain accounted for a quarter of all the pieces exported from the EU to Hong Kong, where they can be mixed with smuggled ivory from recently killed elephants.
EU ivory exports to Hong Kong increased sevenfold to 10,761 pieces last year from 1,572 in 2014. The reasons behind the rise are not fully understood but one theory is that traders are rushing to export as much as possible before new restrictions are imposed.
Hong Kong has pledged to ban ivory trading by 2021 and to stop importing it before that date but it is unclear when the proposed new rules will come into force.
The Born Free Foundation, a wildlife charity, called for Britain and other countries to close a loophole under which antique ivory can be exported to the Far East, where there is huge demand for ornaments and jewellery made from elephant tusks.
Will Travers, president of the organisation, said: “By continuing to permit the export of ivory items into commercial markets such as Hong Kong, the UK is providing an ongoing cover for the illegal ivory trade.
“Creating exemptions . . . offers an ongoing opportunity for ivory laundering and sustaining demand. This undoubtedly means more elephants will die a bloody, brutal and unnecessary death.”
Hugh Fearnley-Whittingstall, the chef and broadcaster, recently demonstrated that ivory on sale in Britain is falsely being labelled as antique.
He bought nine pieces of ivory online and had them radiocarbon dated by an Oxford professor for a forthcoming BBC documentary. Four of the pieces came from elephants whose tusks had grown after 1947.
Poaching has caused the population of African elephants to fall by more than 110,000 in the past decade, according to a report that was published this week.
There are now estimated to be about 415,000 elephants in Africa — down by more than a fifth on 2006.
Last week the Duke of Cambridge spoke of his fear that the African elephant would disappear from the wild because of poaching by the time his daughter turned 25.
Delegates from more than 180 countries are attending an international wildlife trade summit in Johannesburg to discuss rival proposals for loosening or strengthening rules restricting the ivory trade.