Former Prime Minister Baroness Thatcher has been taken to hospital after "feeling faint", the Tory party says.
Baroness Thatcher has been in frailer health
Lady Thatcher, in Downing Street from 1979 to 1990, has suffered from frailer health in recent years but recently held a lavish 80th birthday party.
She will stay overnight at the Chelsea and Westminster Hospital but is expected to return home on Thursday.
Following some minor strokes, she has restricted the number of speaking engagements she undertakes.
Indeed, she spoke only briefly at her 80th birthday festivities in October, attended by the Queen and Tony Blair.
On Tuesday, Lady Thatcher sent her congratulations to David Cameron, who has become the fifth Tory leader since her time in office.
Mr Cameron said on Wednesday: "I was sorry to hear that Lady Thatcher is unwell. I wish her a full and speedy recovery."
A Conservative spokesman said Lady Thatcher was being held in hospital as a precautionary measure.
She began to feel faint on a visit to the hairdressers and was taken to hospital by her bodyguards.
Mark Purcell, of Chelsea and Westminster Hospital, confirmed that Lady Thatcher was admitted to the Fulham Road hospital at about 1700 GMT.
He said: "She has had a number of tests and she will remain in hospital overnight as a precaution.
"She will be assessed by doctors in the morning. Her condition is stable and comfortable and she is now resting."
Daughter Carole - who is still in Australia having won the ITV1 show I'm a Celebrity, Get Me Out of Here - and her twin brother, Mark, who is also overseas, have both been informed.
Lady Thatcher served 11 years as prime minister before her ousting in 1990 - the longest period in the job in the 20th Century.
Her husband, Sir Denis Thatcher, died in June 2003, aged 88.