Queen Elizabeth II's 99-year-old husband Prince Philip left a central London hospital on Tuesday, after four weeks of treatment that sparked fears for his health.

The Duke of Edinburgh, as he is formally known, was seen leaving the private King Edward VII's Hospital in a the back of a black BMW car, wearing a white shirt and a yellow jumper.

Buckingham Palace confirmed his discharge and said he had returned to Windsor Castle, west of London, where had been isolating with the queen, 94, since the start of the coronavirus pandemic last year.

Royal aides said he had been treated for an unspecified infection and had "a successful procedure for a pre-existing condition".

"His Royal Highness wishes to thank all the medical staff who looked after him at both King Edward VII's Hospital and St Bartholomew’s Hospital, and everyone who has sent their good wishes," a statement from the palace read.

The prince is understood to be in good spirits. 

His month-long stay at King Edward VII's Hospital and the state-run St Bartholomew's, also in London, is his longest stint in hospital. 

The duke was transferred to a specialist cardiac unit at St Bartholomew's hospital for a heart procedure before returning to the private hospital on March 5.

He was first admitted on February 16 on the advice of his doctor after he complained of feeling unwell. 

Buckingham Palace said initially that he was expected to stay there for a "few days" but stressed his illness was not related to Covid-19.

Later, the palace said he was being treated for an infection, without specifying further.

Not a racist family

In 2011 Philip was taken to hospital by helicopter after suffering chest pains in the run-up to Christmas. 

He was treated for a blocked coronary artery and had a stent fitted.

Concerns for Philip's health had been high because of his age -- he turns 100 in June -- and came amid the build-up to a landmark interview given by Prince Harry and his wife Meghan Markle accusing the royal family of racism.

The couple sat down with US television host Oprah Winfrey this month for the bombshell interview, in which the couple said they were asked by a member of the royal family what colour their son's skin would be. 

Buckingham Palace has pushed back against claims of racism in a statement by Queen Elizabeth saying "some recollections may vary", over how her grandson and his mixed-race spouse were treated before they stepped back from royal duties.

Responding to questions from reporters about the interview, Harry's brother Prince William told reporters: "We're very much not a racist family".

Ahead of the interview royal commentators said the timing with Philip in hospital was poor.

ITV, which aired the interview in Britain, said it would pull the interview if Philip's health worsened in hospital but ultimately it broadcast the segment.