THE lamprey, regarded as among the most primitive of all living vertebrates, but has proved a favourite with gourmets for centuries.

Studies of fossil remains from more than 300 million years ago have concluded lampreys, which measure from 25cm to 40 cm and use their jawless mouths to suck living tissues from host fish, have changed little.

While the fish, which was considered threatened across Europe in the 1990s, has seen a revival over the past decade, ecologists say it is important to continue conservation work as it plays a vital role in river ecosystems.

Finding lampreys - which create conspicuous spawning grounds known as redds - in a stream indicates oxygen levels, sediment and gravel the river are good.

The fish, which remain a delicacy in south-west Europe, were a popular dish with the upper classes in the Middle Ages. Samuel Pepys mentions them in his diaries and King Henry I is said to have died from eating a surfeit of the fish.

In 2012, due to the decline of lampreys in the UK, residents of Gloucester were forced to import the fish from Canada in order to present the Queen with the lamprey pie it has sent to the Royal household for every coronation or jubilee.