Baroness Thatcher was undergoing tests in hospital yesterday after failing to recover from flu.

The former Prime Minister was said to be “in good spirits” despite doctors deciding last night that she needed to be admitted for assessment.

David Cameron took time out from preparations for the Comprehensive Spending Review to wish Lady Thatcher a “speedy recovery”.

Last week the Tory peer 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 on Tuesday afternoon and aides called in a doctor to assess her condition.

She was taken from her home to the private Bupa Cromwell Hospital in west London shortly afterwards.

After visiting Lady Thatcher on Tuesday night, her son Mark said she was “in good spirits”, and “looked well”.

“She’s been recovering quite well but the doctors wanted to bring her into hospital for some routine tests over the next couple of days,” he told reporters.

“If they go according to plan she’ll be home shortly.”

Asked how she was doing, he said: “Surprisingly good.”

Sir Mark added: “She is in very good spirits, absolutely excellent. It’s entirely precautionary.”

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.”

Lady Thatcher insisted around 150 friends and former colleagues went ahead with her birthday reception at No. 10 last week, despite her absence. Mr Cameron has promised to hold another celebration when she has recovered.

The one-time Iron Lady, who turned 85 last Wednesday, very rarely speaks in public now on medical advice. However, she does still attend a number of high-profile functions.