North Korea has slammed Britain for accusing it of being behind a global ransomware attack that hit the National Health Service, calling the allegation a "wicked attempt" to further tighten international sanctions against Pyongyang.

A third of Britain's public hospitals were affected by the WannaCry worm in May, according to a government report.

Up to 300,000 computers in 150 countries were hit by WannaCry, which seized systems and demanded payment in Bitcoin to return control to users.

Some researchers have pointed the finger at Pyongyang, saying that the code used was similar to past hacks blamed on Kim Jong-un's regime.

Home Office minister Ben Wallace told the BBC last week that London was "as sure as possible" that North Korea was responsible.