Scientists believe they have discovered a new breed of mosquito deep in the London underground railway network.

The insects appear to have mutated from the bird-biting form that colonised the underground when it was built in the last century to a variety that nips rats, mice and maintenance workers.

The scientists say the underground mosquitoes are reluctant to mate with their outdoor cousins, indicating that they have become a separate species -- a process that normally takes thousands of years rather than decades.