The British cyber-security expert who defeated the Wannacry cyber-attack has revealed he briefly thought he might have helped it spread.

The ransomware caused havoc in UK hospitals as well as striking dozens of other organisations across the globe last month.

In an exclusive interview with BBC Horizon, Marcus Hutchins recalled how he halted the blackmailing malware in its tracks.

UK-based viewers can see Horizon: Cyber Attack - The Day the NHS Stopped, on BBC Two at 21:00 BST this Monday.