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Doctors surgeries across Kent are urgently contacting their patients to warn them they cannot book or attend appointments.
A technical problem with the NHS IT system has been blamed as practices cannot access vital information.
Albion Place Medical Practice in Maidstone put out a warning on its Facebook page, saying: "We are currently experiencing technical difficulties with our server.
"We cannot access any of our clinical systems therefore we are unable [to] book appointment or conduct appointments booked for this morning.
"We apologise for an [sic] inconvenience caused and will continue to keep you updated."
Thames Avenue Surgery in Rainham made a plea to patients to bear with its reception staff.
It posted: "Please don't have a pop at our reception staff as had already happened – it's not our fault and it makes for stress all round."
Marden Medical Centre which serves the village of Marden, near Maidstone, posted: "This is a Kent wide problem with computers at the moment.
"Please do not call to make an appointment until late morning."
As of just before 11am, some surgeries started to report the issue had been resolved.
Newton Place Surgery in Faversham took to Facebook to say: "Thank you for your patience. We’re now back up and running."
While it appears appointments at acute hospitals have not been affected, the issue is impacting some phone lines.
"Please accept our apologies for any inconvenience."
Kent Community Health NHS Foundation Trust said in a Facebook post: "If you are trying to contact our community hospitals, some of our phone lines are affected by a national network issue.
"We hope this will be solved soon and would like to apologise for any inconvenience."
And Medway NHS Foundation Trust said: "Due to technical issues with our phone lines, some patients may have trouble contacting our outpatient phlebotomy services at Medway Maritime Hospital and Sheppey Community Hospital.
"Please contact us on email at medwayft.phlebotomyadmin@nhs.net. Please accept our apologies for any inconvenience."
Patients have been advised to call 111, or 999 if it is an emergency.
An NHS Digital spokesman said: "There was a temporary issue affecting internet connectivity at a small number of healthcare sites, mainly GP practices and community clinics, earlier today. This began at 2am during planned maintenance and has now been resolved by the supplier."