Baroness Thatcher was said last night to be undergoing "precautionary" medical checks at St Thomas's Hospital in London.
The reason for the treatment was not clear. But a Conservative Party spokeswoman said: "We have been in touch with her office and we wish her well."
Friends of Lady Thatcher, 82, have been concerned about her health in recent months, and she has scaled down her public appearances on the advice of her doctors.
Last September she made a well publicised visit to No 10, where she spoke to Gordon Brown – then riding high in the polls.
In January the Tory leader David Cameron presented the former prime minister with a lifetime achievement award at the Morgan Stanley Great Britons Awards in central London. Then last month Lady Thatcher, who served as prime minister from 1979 to 1990, attended the unveiling of a statue of herself in the House of Commons, which she described as "an honour", and joked that "I might have preferred iron but bronze will do," in a reference to her nickname as "the Iron Lady".
In 2005, Lady Thatcher spent a night on the coronary ward at the Chelsea and Westminster Hospital in London, before leaving with a "clean bill of health" and telling reporters: "I'm fine, I'm fine. I feel fine."
In 2002 she suffered a number of small strokes. At that time, her office issued a statement announcing she was cancelling public engagements.
A spokesman said: "Over recent months Lady Thatcher has suffered a number of small strokes ... [Her] doctors have told her that these can neither be predicted nor prevented. They have therefore told her to cut back her programme at once and in particular to avoid the undue strain that public speaking places on her.
"With great regret, she has decided to abide by this advice and to cancel all her speaking engagements."
Last night Sky News's political editor Adam Boulton said he had spoken to friends who had recently hosted Lady Thatcher, who didn't know "where she was or who they were or why she was there".Reuse content