London - Former British prime minister Margaret Thatcher will have to undergo an operation to insert a pin into her broken upper arm following a fall, her spokesman said Thursday.

The 83-year-old former "Iron Lady" was admitted to London's Chelsea & Westminster hospital last Friday, where doctors have now found that the injury she sustained is not healing as quickly as they would like.

She was initially expected to leave the hospital on the day she was admitted, but doctors extended the stay for what they said was a "precaution."

"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," her spokesman Mark Worthington said.

Earlier today, her son Mark said his mother was "very well and alert."

But, acknowledging the family's concern, he added: "I think every son is always worried about his mother's health whatever her age."

Thatcher has become increasingly frail in recent years, following a series of minor strokes which led doctors to ban her from public speaking in 2002.

Her daughter Carol, a twin to Mark, last year revealed that her mother was battling with dementia. (dpa)