"We can confirm that Baroness Thatcher has been admitted to St Thomas' Hospital and is expected to remain in hospital overnight for observation," a statement released by the hospital said.
"Her condition is stable and she is speaking to the medical staff who are caring for her.'"
Thatcher, Britain's first female prime minister, has often appeared at private functions in recent years, but makes few public statements.
Earlier this year, she urged members of her Conservatives party to "hold firm to their beliefs" after she was honoured with a statue at the party's headquarters.
Thatcher, nicknamed the Iron Lady, was prime minister from May 1979 until her resignation in November 1990.
She was the first leader to win three elections in a row.
Thatcher dominated British politics in the 1980s and was a firm supporter of Ronald Reagan, the former US president.
She was given a clear bill of health by doctors in December 2005 since spending a night in a London hospital for tests after feeling faint.
In March 2002, she cut back her workload after doctors said she had suffered a series of strokes.