Former prime minister Lady Thatcher will have a "routine" operation to insert a pin into her broken upper arm following a fall.
The 83-year-old has been recovering in hospital since last week after breaking her upper left arm but doctors have decided that the injury is not healing as quickly as they would like.
Spokesman Mark Worthington said on Thursday: "Lady Thatcher is to undergo a routine surgical procedure to insert a pin into the bone of her upper arm in order to assist the healing process. Therefore she will be remaining in hospital for the time being."
Lady Thatcher was initially expected to leave hospital last Friday until doctors decided to keep her in as a precaution, but a spokesman stressed that she was otherwise well.
Earlier on Thursday, son Sir Mark Thatcher said his mother was "very well", and "alert and talking about Ladies Day at Ascot".
He acknowledged the family's concern saying: "I think every son is always worried about his mother's health whatever her age."
He said his twin sister Carol had not visited their mother because she was recovering from a recent operation on her eye.
"But she has been in contact with me on a regular basis, she just can't make it down here."
Lady Thatcher has suffered a number of health scares in recent years, including a series of minor strokes. In March last year she spent a night in hospital after feeling unwell while having dinner with friends.
Lady Thatcher cut down her commitments - all but stopping speaking in public - on the advice of doctors several years ago but she continues to attend ceremonial functions.