Mrs Thatcher, 87, was admitted to hospital on Thursday and expected to be kept in overnight last night after having a growth removed from her bladder.
Sources suggested she may remain in hospital to recuperate for a few days. Her daughter Carol has been to visit her.
"She is absolutely fine," the spokeswoman said of the ex-premier, who has suffered some ill health in recent years.
Lord Bell, a friend of the Baroness, said: "She's very tough. The doctors are perfectly happy. They say the operation has been completely satisfactory."
Baroness Thatcher 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.
Two years ago she missed an 85th birthday party thrown for her by Prime Minister 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.
She had a series of minor strokes in 2002 and was advised by doctors to stop making public speeches.
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.