Queen Elizabeth II of the UK has been placed under constant medical supervision by her doctors, Buckingham Palace has announced.

The update on the 96-year-old monarch's condition comes less than 24 hours after it was confirmed she was unable to attend a Privy Council meeting, a body of advisors on government issues, for health reasons.

The longest-serving current monarch did not return to London despite the announcement of a new Prime Minster, instead opting to stay at her Scottish residence Balmoral for what her doctors described as "mobility issues."

The Queen was pictured with the new leader of the UK government, Liz Truss, on Tuesday but since the announcement of the deterioration of her condition, the monarch's eldest son and heir Prince Charles is heading to Balmoral, as is his own first-born son the Duke of Cambridge.

"Following further evaluation this morning, the Queen's doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision," a statement from Buckingham Palace read. "The Queen remains comfortable and at Balmoral."

The monarch has been suffering from what Buckingham Palace has called "episodic mobility problems" since the end of last year, which have forced her to cut back on engagements since then and reduce her public appearances.

The Queen has been at Balmoral on a summer break since July. Boris Johnson formally resigned there on Tuesday as the UK Prime Minister and shortly afterwards, Liz Truss was appointed his successor and the 15th different premier of Elizabeth's reign.

Truss said: "The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime.

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