A screengrab of an 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.”

A second doctor tweeted: “Massive NHS hack cyber attack today. Hospital in shut down. Thanks for delaying emergency patient care & endangering lives. Assholes.”

East and North Hertfordshire NHS Trust, one of those affected, said in a statement: “Today, the trust has experienced a major IT problem, believed to be caused by a cyber attack.

“Immediately on discovery of the problem, the trust acted to protect its IT systems by shutting them down; it also meant that the trust’s telephone system is not able to accept incoming calls.

“The trust is postponing all non-urgent activity for today and is asking people not to come to A&E.”

Blackpool Hospitals, also affected, tweeted: “Our computer systems are experiencing problems and we are working hard on a solution. We will update you as soon as possible.”

An NHS source told the Evening Standard the attack “seems to be growing” with more incidents of hospitals across Britain facing IT problems being reported.

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“At approximately 12:30pm 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,” an NHS IT worker said in a message to a Guardian reporter.

“A bitcoin pop-up message had been introduced onto 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 staff to shut down all PCs in the trust and await further instructions.”