Baroness Thatcher is recovering in hospital after undergoing surgery to remove a growth on her bladder.
The former Prime Minister is "absolutely fine" after the minor operation, a spokeswoman said.
The 87-year-old was admitted to hospital on Wednesday after complaining of pains. She is expected to remain there while she recovers.
Former Thatcher adviser Lord Bell said: "Following an examination in hospital on Wednesday Mrs Thatcher had a minimal invasive operation to remove a growth in her bladder.
"The operation was completely satisfactory.
"She will stay in hospital to recuperate for a few days and then she will return home. Her daughter Carol was with her in hospital.
"We will make no further comment."
Baroness Thatcher has suffered a number of health scares in recent years, including a series of minor strokes.
She was not well enough to join the Queen for a lunch with former and serving prime ministers as part of the Diamond Jubilee this summer.
And two years ago she missed an 85th birthday party thrown for her by David Cameron at 10 Downing Street.
In October she was sufficiently well, however, to mark her 87th birthday with lunch at a restaurant in London's exclusive St James's district with her son Mark and his wife.
Her health was thrust into the global spotlight this year when Meryl Streep starred in a controversial Hollywood film about her.
The Iron Lady drew criticism from Mr Cameron and others for concentrating on the dementia she has suffered after a series of small strokes. Streep won an Oscar for the role.
Baroness Thatcher was the first female prime minister - between 1979 and 1990 - and first female leader of the Conservative Party.