The IT systems of NHS sites across the country on Friday appear to have been simultaneously hit, with a pop-up message demanding a ransom in exchange for access to the PCs, the Guardian reported.

NHS England said it was aware of the problem and would release more details soon.

Affected hospitals include those run by East and North Hertfordshire NHS trust, Barts Health in London, Essex Partnership university NHS trusts, the university hospitals of Morecambe Bay NHS foundation trust, Southport and Ormskirk hospital NHS trust and Blackpool teaching hospital NHS foundation trust.

One NHS IT worker told the Guardian: "At approximately 12.30 p.m. we experienced a problem with our email servers crashing. Following this a lot of our clinical systems and patient systems were reported to have gone down."

"A bitcoin virus pop-up message had been introduced on to the network asking users to pay $300 to be able to access their PCs. You cannot get past this screen.

"This followed with an internal major incident being declared and advised all trust staff to shut down all PCs in the trust and await further instructions," the worker said.

The worker said that the hack was affecting the east of England and number of other trusts.

"This is the largest outage of this nature I've seen in the six years I've been employed with the NHS," the worker said.

GP surgeries across Liverpool and parts of Greater Manchester also appeared to have been affected by the cyber-attack.

Doctors have been posting on Twitter about what has been happening to their systems.

A screengrab of a instant message conversation circulated by one doctor says: "So our hospital is down … We got a message saying your computers are now under their control and pay a certain amount of money. And now everything is gone."

IT specialists were working to resolve the problem as quickly as possible, a statement from the NHS Trust said.