Fears grew on Thursday for Queen Elizabeth II after Buckingham Palace said her doctors were "concerned" for her health and recommended that she remain under medical supervision.

The 96-year-old head of state — Britain's longest-serving monarch — has been dogged by health problems since last October that have left her with difficulties walking and standing.

On Wednesday, she pulled out of a planned meeting with her senior political advisors, after being told to rest.

The previous day she held audiences at her Scottish Highlands retreat, Balmoral, with outgoing prime minister Boris Johnson and appointed his successor, Liz Truss.

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

"The Queen remains comfortable and at Balmoral," the palace added.

The palace statement about the queen's health is highly unusual and comes after she has looked visibly frailer in recent months, and a succession of withdrawals from public engagements.

She has taken to walking with the help of a stick and was also seen earlier this year at the Chelsea Flower Show touring the site in a motorized buggy.

In February she was laid low by a bout of Covid, which she admitted afterwards had left her "exhausted".

'Deeply concerned'

The queen's eldest son and heir Prince Charles, 73, and his eldest son, Prince William, 40, were heading to Scotland, according to their Clarence House and Kensington Palace offices.

Officially, the palace has said only that the queen has been suffering from "episodic mobility problems" but given no further details.

She spent an unscheduled night in hospital in central London in October 2021 and was advised to slow down.

Moments before Thursday's announcement, notes were passed to Truss and senior members of her team in parliament, prompting them to leave the chamber.

Truss tweeted almost immediately afterwards: "The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime," she added.

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

Archbishop of Canterbury Justin Welby, the highest-ranking cleric in the Church of England that the queen heads, said the queen was in his prayers.

"May God's presence strengthen and comfort Her Majesty, her family, and those who are caring for her at Balmoral," he tweeted.

The queen is in her Platinum Jubilee year, marking 70 years since she succeeded her father king George VI in 1952.

Four days of public events were held to mark the record-breaking occasion, but she made only two appearances to acknowledge the huge crowds in central London.