A British cyber expert has told how he jumped around in excitement after discovering a kill switch which halts the global spread of the malicious software currently infecting the NHS and organisations in more than 100 countries.

The 22-year-old researcher has been hailed as an 'accidental hero' online.

The anonymous blogger discovered that upon infecting a new computer, the virus contacts a remote web address and only starts taking files hostage if it finds that address unreachable.