Doctors are concerned about the health of Britain's Queen Elizabeth and have recommended the 96-year-old remains under medical supervision, Buckingham Palace says, with immediate family travelling to be by her side.

The Queen, Britain's longest reigning sovereign and the world's oldest monarch, has been suffering from what Buckingham Palace has called "episodic mobility problems" since the end of last year.

She has been forced to cut back on her public engagements since then and on Wednesday cancelled a planned virtual meeting with senior ministers after being advised to rest by her doctors.

The previous day she had been pictured appointing Liz Truss as the country's new prime minister at her Scottish home, Balmoral Castle.

"Following further evaluation this morning, The Queen's doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision," the palace said in a statement on Thursday.

"The Queen remains comfortable and at Balmoral."

A palace source said immediate members had been informed and played down speculation that the monarch had suffered a fall.

"Their Royal Highnesses The Prince of Wales and the Duchess of Cornwall have travelled to Balmoral," a Clarence House spokesperson said.

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Prince William is also travelling to Balmoral, a Kensington Palace source confirmed.

Truss said the whole country would be deeply concerned by the news.

"My thoughts - and the thoughts of people across our United Kingdom - are with Her Majesty The Queen and her family at this time," she said.

Lindsay Hoyle, the speaker of the House of Commons, interrupted an energy debate in parliament to say he sent his best wishes to the monarch.

The Queen's health has been scrutinised in recent times, with Her Majesty having to pull out of a number of events.

While she did appear on the Buckingham Palace balcony for the Platinum Jubilee on June 2, she did not attend the Platinum Jubilee Service of Thanksgiving the following day due to "discomfort" the celebrations had already caused.

It came after she was seen using a walking stick for the first time at a major engagement in October 2021.

She also pulled out of a rumoured appearance at the Royal Windsor Horse Show with little notice in May.

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