The pest, which entered the Tube system 100 years ago, is evolving into a new species with an appetite for mammalian blood.

Scientists say the insect has evolved so fast that the difference between the overground and underground forms is as great as if they had been separated for thousands of years.

Commuters make a good snack

They have even found genetic distinctions between mosquitoes living on separate lines.

Culex pipiens probably ended up in the tunnel network when it was being dug during the last century.

But faced with a shortage of birds it began to look for an alternative blood source.

The insect now lives off rats, mice, and sometimes humans, according to an article in BBC Wildlife Magazine.

During World War II the pest became notorious for attacking Londoners sheltering from Hitler's bombs in the Underground.

Researchers Kate Byrne and Richard Nichols, from London University's Queen Mary and Westfield College, found underground and overground populations of the mosquito were genetically quite different.

Rats: An alternative delicacy

Most attempts to cross the two types of mosquito failed, suggesting they were well on the way to becoming completely separate species.

The genetic differences between mosquitoes living on different tracks were attributed to draughts created by trains dispersing the insects along, but not between, Tube lines.

Roz Kidman Cox, Editor of BBC Wildlife Magazine, said: "It's a remarkable story of evolution.