Mrs Thatcher was known as the “Iron Lady” during her time Conservative prime minister from 1979 to 1990.
A Conservative Party spokesman said that Mrs Thatcher, 80, would spend the night in hospital as a precaution and doctors believed she would be able to leave in the morning.
"She was not feeling very well this afternoon. They took her into hospital and they are running a few tests," she said.
BBC television said Thatcher had felt unwell during an appointment with a hairdresser, and her bodyguards took her to the hospital.
In recent years, she has suffered a series of minor strokes and in 2002 her
doctor told her to stop accepting public speaking engagements, citing health
grounds.
Chelsea and Westminster Hospital spokesman Mark Purcell confirmed that Thatcher was admitted to the Fulham Road hospital at about 1700 GMT.
"We can confirm that Lady Thatcher was brought to the accident and emergency department earlier this evening at about 5 pm," he said.
"She has had a number of tests and she will remain in hospital overnight as a precaution," he added.
"She will be assessed by doctors in the morning. Her condition is stable and comfortable and she is now resting."
Mrs Thatcher, Britain's first female prime minister, was known as a strong leader and an astute Parliamentary tactician according to Britannica.com.
In 1982 she ordered British troops to the Falkland Islands to retake them from Argentina.
She also took a strong stand against the trade unions during the 1984 miner's strike, and moved Britain toward privatisation.
In 1989, she introduced the controversial community poll tax.
She was forced to resign in 1990 as her cabinet was divided over major issues including the European Community.
She became Baroness Thatcher of Kevesten in 1992 and is a member of the House of Lords.