David Cameron wished Baroness Thatcher a "speedy recovery" tonight after she was taken to hospital suffering from an infection.
Last week the Tory former prime minister was forced to withdraw from a Downing Street reception to mark her 85th birthday 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 this afternoon. She was taken from her home to hospital shortly afterwards, and is expected to stay overnight.
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."