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Forgein molluscs and killer shrimps have colonised Britain's rivers so extensively that some already look like the Caspian Sea, academics have said.

A Cambridge University study has found that invasive species from Turkey and Ukraine are overwhelming native British species.

Travelling to our shores on ship hulls or clinging to ornamental plants imported for our gardens, the animals are creating eco-systems that will support other foreign animals.

'Pink peril': The killer shrimp, pictured, is among the invasive species that are colonising Britain's rivers. The animals kill and feed on native British shrimps

Five of the most invasive freshwater creatures from the Black Sea, Caspian Sea and Sea of Azov are now in Britain - including the quagga mussel, found just two weeks ago in the Wraysbury River near Heathrow airport.

Researchers say that, with at least ten more of these high-risk species established just across the channel in Dutch ports, Britain could be on the brink of what they describe as an 'invasional meltdown'.

They have called for greater monitoring on areas at most risk - the lower reaches of the Thames, Severn, Great Ouse and the Norfolk Broads.

Their dire warning, published on Monday in the Journal of Applied Ecology, says the issue is vital for 'national biosecurity'.

Greatest threat: The invader scientists are most concerned about are the quagga mussel, pictured, which alarmingly was first discovered in the UK just two weeks ago

Study author Dr David Aldridge said: 'In some parts of Britain the freshwater community already looks more like the Caspian Sea.

'The Norfolk Broads, for example, typically viewed as a wildlife haven, is actually dominated by Ponto-Caspian zebra mussels and killer shrimps in many places.

'Pretty much everything in our rivers and lakes is directly or indirectly vulnerable.

Tiny killer: The quagga mussels, discovered in a reservoir near Heathrow, will smother and kill native mussels, affect water quality and block pipes

'The invader we are most concerned about is the quagga mussel, which alarmingly was first discovered in the UK just two weeks ago. This pest will smother and kill our native mussels, block water pipes and foul boat hulls.'

Other invasive species include the demon shrimp, bloody red shrimp and the killer shrimp - dubbed the 'pink peril'.

Each of these species kill and feed on native British shrimps.

But more alarmingly, the foreign invaders act as a gateway for further invasive species, because they kill the competition and in some cases create an eco-system which is favourable for other invaders to thrive.

The zebra mussel, for example, provides an ideal home for invasive shrimps which have striped patterns to blend in with the mussels' shells.

The shrimps, in turn, provide food for larger invaders such as goby fish. Gobies have now made their way down the Rhine, one of the main corridors to Britain, with populations exploding in the waterways of western France over the last few years.

The invading gobies eat native invertebrate and displace native fish such as the already threatened Bullhead.

Dr Aldridge said: 'If we look at The Netherlands nowadays it is sometimes hard to find a non-Ponto-Caspian species in their waterways.'

Sarah Chare, deputy director of fisheries and biodiversity at the Environment Agency, said: 'Invasive species – such as the quagga mussel – cost the UK economy in excess of £1.8 billion every year.

'The quagga mussel is a highly invasive non-native species, affecting water quality and clogging up pipes.'