Baroness Thatcher is in a "stable" condition, a spokeswoman for St Thomas' Hospital said today.
The former Prime Minister, 82, is said to be undergoing medical checks at the central London hospital.
The spokeswoman said last night Lady Thatcher, pictured, was in hospital for observation.
In a statement, she said: "We can confirm that Bar
oness Thatcher has been admitted to St Thomas' Hospital and is expected to remain in hospital overnight for observation.
"Her condition is stable and she is speaking to the medical staff who are caring for her."
A Conservative Party spokeswoman said: "We have been in touch with her office and we wish her well."
It was reported Lady Thatcher was taken to hospital by car.
Lady Thatcher very rarely speaks in public, following advice from her doctors, but she does still sometimes say a few words at functions.
Friends say she is lucid most of the time but occasionally drifts off in the middle of conversations because of difficulties with her short-term memory.
Earlier this month she urged Conservatives to "hold firm to their beliefs" as she was honoured with a statue at the party's HQ.
On her first visit to Conservative Central Office since the party moved to 30 Millbank in Westminster last year, Lady Thatcher met staff and unveiled a statue of herself in the reception area.
Lady Thatcher was Prime Minister from May 1979 until her resignation in November 1990. She was Britain's first woman prime minister and the first leader to win three elections in a row.
Her supporters believe she put the drive back into Britain and while many saw her as a divisive force, history will almost certainly proclaim her as one of the greatest British peacetime leaders.
In March 2002 she cut back her workload after doctors said she had suffered a series of strokes.
