Former Prime Minister Baroness Thatcher has been admitted to hospital to be treated for an infection.
It is understood she has not fully recovered from an illness that prevented her attending Downing Street last week to mark her 85th birthday.
Her son Mark said she was "in good spirits" and her admission was "entirely precautionary". She is expected to return home in a few days.
Prime Minister David Cameron wished Baroness Thatcher a "speedy recovery".
And her former aide, the Conservative MP Gerald Howarth, said: "We all wish her well, and are sorry we did not see her last week."
Baroness Thatcher was prime minister from 1979 to 1990.
Muscular problem
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Absolutely excellent, surprisingly good”
End Quote Mark Thatcher Asked how his mother Baroness Thatcher was feeling
She was taken to Cromwell Hospital, west London, on Tuesday evening in what has been described as a "routine admission".
The BBC understands that following a bout of flu, she has been suffering from a condition affecting her muscles, which has made it difficult for her to move around.
Sources say the condition is "eminently treatable".
As he left the hospital, Mr Thatcher said doctors wanted to keep her in hospital for a couple of days to carry out some routine tests.
"If they all go according to plan she'll be home shortly," he said. "It's entirely precautionary at this stage."
Asked how his mother was, he added: "Absolutely excellent, surprisingly good, looks well. Still awake at this time of night which is good."
In 2005, the former PM was advised by doctors that she should not make public speeches in the wake of some minor strokes.
But she still attends some public functions, including a recent address by the Pope during his state visit to the UK.
In March 2008, she was taken ill during a dinner in Westminster and spent the night in hospital as a precaution, but was later given a clean bill of health.
