Britain's Prince Philip has left the private London hospital where he had spent nearly two weeks to have heart tests and treatment at another hospital, Buckingham Palace said.

Queen Elizabeth's 99-year-old husband was taken from King Edward VII's hospital to St Bartholomew's Hospital in the City of London.

Doctors there "will continue to treat him for an infection, as well as undertake testing and observation for a pre-existing heart condition", the palace added.

"The Duke remains comfortable and is responding to treatment, but is expected to remain in hospital until at least the end of the week."

The Duke of Edinburgh was admitted to King Edward VII's hospital on 16 February as a "precautionary measure" and on the advice of his doctor, after feeling unwell.

But the palace was quick to point out that his admission was not related to Covid-19, and later said he was being treated for an undisclosed infection.

He and the queen, 94, both had the first dose of a Covid vaccination on 9 January because of their age, which puts them in one of the most vulnerable categories.

An ambulance was seen leaving the King Edward VII's hospital late this morning, driving off past banks of photographers and camera crews.

But a police van - and tinted windows in the ambulance - blocked sight of who was inside the vehicle, which left from a rear entrance.

Police on guard outside St Bartholomew's Hospital in London

Umbrellas were also held up to conceal the gap between the ambulance and the hospital doors.

Prince Philip will be 100 on 10 June. His already lengthy stay in hospital has raised concerns because of his age, even though he was said to have been comfortable and responding to treatment.

His youngest son, Prince Edward, said last week his father was feeling "a lot better" and was looking forward to returning home.

He thanked members of the public for sending messages of support.

Last Monday, Philip's grandson, Prince William, said the duke was "okay" and being closely monitored by medical staff.

Two days earlier, Philip's eldest son Prince Charles visited him in hospital but made no comment afterwards.

Philip was able to walk into the hospital unaided when he first was first admitted and arrived using private transport, rather than an ambulance, a source said at the time.

The former naval officer was treated at the same hospital in August 2012 after a recurrence of a bladder infection that laid him low during the queen's diamond jubilee celebrations.

The previous year, he had a stent fitted after suffering a blocked coronary artery.

St Bartholomew's Hospital, near St Paul's Cathedral, is internationally renowned and a centre of excellence for cardiac and cancer care.

Barts Heart Centre is Europe's largest specialised cardiovascular service, according to the hospital's website.