EMIS PCS, the management system for doctors' surgeries, was unavailable for use to hundreds of doctors for much of Wednesday and at least some of Thursday.
A practice manager from a south London surgery who requested anonymity told us: "Our system has been down all day.
"All the surgery appointments are on the clinical system and chaos has ensued in large medical practices where they don’t keep paper records or paper appointment sheets, with patients unable to access their appointments or have repeat prescriptions fulfilled or their medical notes accessed or updated during today."
Managers will spend much of today sorting out the backlog of paperwork.
Sean Riddell, chief executive of EMIS, said: "We had an outage in our data centre yesterday morning for a number of hours, which unfortunately affected around 200 EMIS PCS practices (out of a total of 5,500 EMIS practices) in certain areas of the country.
"EMIS moved swiftly to address this and staff worked as quickly as possible to restore normal service. A significant number of affected practice systems were restored by around 1pm, the remainder by 3pm.
"This was an extremely rare incident for EMIS. I would like to apologise to all the practices affected and reassure them that everything was done to rectify it as quickly as possible."
EMIS is in the midst of moving surgeries onto an online system. ®