IT services were wiped out at Guy’s and St Thomas’ hospitals as temperatures surged beyond 40C during a record-breaking heatwave on Tuesday, July 19.
Chief operating officer John Findlay said it was "the most complex" incident at Guy’s and St Thomas’ NHS Foundation Trust "in a very, very long time".
He also warned that it would take both hospitals a long time to recover, the BBC reported.
In an internal Zoom call with staff, he said the trust had to switch to paper which “slowed everything down phenomenally”.
"Although it sounds straightforward to move to a paper hospital clearly that is far from the case when you’ve got issues going on for an extended period of time," he said.
"It has slowed everything down phenomenally. It’s meant people have had to work incredibly hard."
He added: "That whole manual process of moving pieces of paper around the organisation, recording the interactions with patients, just fundamentally knowing what is going on and what has happened, is clearly a massive challenge.
"The longer it goes on, the greater that challenge is. I absolutely recognise the significance of this and the impact that it has had on people."
The systems have since been fixed but some problems remain and could continue into next week, chief executive Ian Abbs said.
"The past week has been exceptionally difficult for everyone at Guy’s and St Thomas’," Mr Abbs explained.
"The first thing is that I want to make a heartfelt and unreserved apology for what has happened. This is not what we expect or want ever to happen.
"This has been one of the most serious incidents we’ve ever had to deal with, certainly in my long memory at Guy’s and St Thomas’."
On Thursday, Chief Executive Professor Ian Abbs issued a further apology on the foundation trust’s website, saying: “My executive colleagues and I want to extend a heartfelt apology to all those who have been affected by the extremely serious IT problems that we have been experiencing.
“That apology is to our patients, their families and our local communities for the impact this incident may have had on their care; and also to our colleagues, for the significant impact that this has had on their ability to deliver the high quality care to which we always aspire.”
Guy’s and St Thomas’ NHS Foundation Trust has been contacted for comment.