Baroness Thatcher has been admitted to hospital after failing to recover from an infection.
The former Conservative prime minister was forced to withdraw from a Downing Street reception to mark her 85th birthday last week after coming down with flu. Friends said she was still being troubled by the illness, and aides called in a doctor to assess her condition yesterday afternoon. She was then taken from her home to Cromwell Hospital in West London.
A spokesman for Baroness Thatcher told the BBC last night that her condition is not serious and that she is expected to return home within a few days.
A Downing Street spokesman said: "We understand from Lady Thatcher's office that she has been admitted to hospital for precautionary tests following her recent bout of flu.
"The Prime Minister wishes Lady Thatcher a speedy recovery."
Gerald Howarth, the Conservative MP and a former aide to Lady Thatcher, added: "We all wish her well, and are sorry we did not see her last week."
Baroness Thatcher, who was Britain's first female prime minister, holding the office between 1979 and 1990, has experienced a number of health problems in recent years.
She suffered a series of mild strokes in late 2001 and 2002, after which she cut back on public appearances and later cancelled her speaking schedule following advice from doctors.
She was taken to hospital in June 2009 when she broke her arm in a fall at her London home.
But Baroness Thatcher has still been able to attend public events, including an address by Pope Benedict during his recent visit to the UK.
After being forced to pull out of last week's birthday celebrations, she wrote a letter of apology to the 150 friends and colleagues who attended.