The UK is trading millions of eels with Russia under a programme that is risking national security, campaigners have warned.

Large numbers of critically endangered glass eels are being transported annually from the Bristol Channel to Kaliningrad under the premise of a conservation programme.

However, wildlife campaigners say the justification is cover for the eels to be held and eventually sold on for consumption.

The trade has risen significantly in recent years, from half a million eels in 2022 to three million last year, with proposals for it to increase fivefold to 15 million.

Two dozen campaign groups have written to The Telegraph calling for the environment department to focus on conservation programmes that keep the European eels, a protected red-listed species, in Britain.

The letter states: “Even at times of peace, it would be irresponsible to propose a trade where monitoring procedures are compromised.

“In the current geopolitical climate, trade with Russia is also surely an issue of national security.”

Signatories include Andrew Kerr, the chairman of the Sustainable Eel Group, Hugh Fearnley-Whittingstall, a leading chef, and Chris Packham, the conservationist.

Eels are born in the Sargasso Sea and make their way in their juvenile state to the UK, where they mature in rivers before heading out to respawn.

However, experts have found that less than 3 per cent of those in the River Severn make it back out to sea, far below the target of 40 per cent needed to recover global stocks.

The low success rates are blamed on the decline of eel habitats caused by factors related to agriculture, industry, housing estates and pollution.

Advocates of eel transport programmes say sending the fish to Kaliningrad helps boost numbers because it is easier for them to reach the sea from there.

However, opponents fear there is a high risk that the eels are being held and matured in Russia before eventually being sold on to markets such as Asia for consumption.