Former Premier appears frail but displays old determination

The former Prime Minister appeared unsteady on the steps of her Belgravia home in central London but did manage a brief wave and a smile to the waiting media before returning inside.

Lady Thatcher was admitted to the private Cromwell Hospital in west London almost a fortnight ago with an infection following a bout of flu.

She entered hospital on October 20 for tests after failing to shake off the illness, which had already forced her to miss her 85th birthday party in 10 Downing Street. The gathering of around 150 friends and colleagues went ahead without her, at her insistence, and she apologised for her absence – quipping "the Lady's not for returning".

Her spokesman said she had now been given the all-clear by doctors.

Prime Minister David Cameron sent his "warmest wishes" and is to find a new date to welcome her back to No 10 to celebrate her birthday.

Mr Cameron paid tribute to Baroness Thatcher in his recent party conference speech, describing her as the "greatest peacetime" Prime Minister of the 20th century.

Medical experts said Lady Thatcher – voted the world's most influential woman in a weekend poll – was likely to have stayed at the private hospital for a "health MoT".

Her son, Sir Mark Thatcher, said a few days after she was admitted she was "chirpy" and had been talking about the Spending Review, despite being slightly sedated at times.

In 2005, the former PM was advised by doctors she should not make public speeches in the wake of some minor strokes.

But she still attends some public functions, including an address by the Pope during his state visit to the UK.

Lady Thatcher, who was in Downing Street between 1979 and 1990, was also admitted to hospital in June last year when she broke her arm in a fall.