Portuguese and Spanish fishing vessels have created a 'wall of death' of fishing lines, decimating Britain's shark population.

Lines 60 miles long and bristling with hooks have been laid across routes taken by sharks as they migrate across the Atlantic, in and out of British waters.

It is estimated that between 3 and 4 million blue and mako sharks, the main types found in UK waters, are being killed by the nets annually.

Researchers at the Marine Biological Association first became aware of the scale of the slaughter after tagging 100 sharks, then comparing the routes they took.

"We found that the sharks are congregating where warm and cool currents meet. These are highly productive areas that attract fish – and that attracts sharks too," David Sims, professor of marine ecology at the MBA, told the Sunday times.

"However, it also attracts fishing vessels and we found many long lines laid in exactly the places where sharks concentrate. It is a wall of death for sharks."

The research helps explain dwindling shark numbers off the coast of Cornwall, where tourists and shark anglers used to catch thousands of sharks annually, and now only catch hundreds.

With tuna overfished and numbers declining, the unregulated shark fishing industry has expanded rapidly, with 100 million sharks killed globally every year to supply the growing market for shark fins in the far-east, where they are used in soup and are believed to have medicinal properties.

With both mako and blue sharks not starting to breed until 18 and six years respectively, experts say that both breeds are particularly vulnerable to overfishing, with young sharks killed before they can have offspring.

Sims said: "These are awe-inspiring animals but it is open season on sharks. We should hit the panic button right now rather than in 10 years' time when it could be too late."