North Korea said Britain was “despicable” in its attempts to “damage” Kim Jong-un’s reputation by associating it with the cyber-attack.
Last week Westminster said it believed “quite strongly” Pyongyang was responsible for the ‘WannaCry’ ransomware attack.
The attack in May hit countries across the globe, including in the UK where NHS computers were frozen and some services were forced to operate as emergency-only.
However, following accusation from Theresa May’s security minister Ben Wallace, North Korea has responded furiously.
A statement from the propaganda news agency KCNA slammed the allegation and warned Britain to “seriously reflect” on what was being claimed.
The statement said: “The UK has made another attempt to incriminate the DPRK as a cyber-criminal.
“The DPRK has clarified our principled stand, on every opportune occasion, to oppose terrorism in all its forms and manifestations.”
The statement even expressed sympathy to those affected and said North Korea would never attack the NHS due to their “priority” for healthcare.
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Kim’s mouthpiece added: “We have expressed sincere sympathy and condolences to the UK when its citizens fell victim to terrorist attack.
“It does not make any sense that the DPRK which gives priority to health would carry out a cyber-attack on the UK health service.”
The statement concluded with a warning which leaves little to the imagination and will surely set alarm bells ringing in Downing Street.
It said: “The UK, if it is a law-governed state, should seriously reflect on its despicable moves to damage the image of the DPRK.
“This is an act beyond the limit of our tolerance.”
Mr Wallace said the Government was almost certain Pyongyang was behind the crippling attack.
He said: “North Korea was the state that we believe was involved in this worldwide attack.”
“They are potentially linked to other attacks about raising foreign currency.”
During the attack affected computer users could regain control of their systems by sending hundreds of pounds worth of Bitcoin to internet addresses.
READ MORE: What is the WannaCry cyber attack?
A red warning page said: “We guarantee that you can recover all your files safely and easily. But you have not so enough time.
“You can try to decrypt some of your files for free. If you want to decrypt all of your files, you need to pay.
“You only have three days to submit the payment. After that the price will be doubled.
“Also, if you don’t pay in seven days you won’t be able to recover your files forever.”