White House Spokesperson John Kirby said the following a short time ago during a briefing:

"His and the first lady's thoughts are solidly and squarely with the Queen today and her family."

The ABC's Europe Correspondent Nick Dole reports from London.

This picture captured by Russell Cheyne from Reuters captured the Duke of Cambridge, the Duke of York and the Duke of Kent arriving at Balmoral.

Per BBC Scotland journalist Linda Sinclair, Prince William, Prince Edward, Sophie Wessex and Prince Andrew have just arrived at Balmoral Castle.

A spokesperson for the UK's newly sworn in Prime Minister Liz Truss says she has no current plans to travel to Balmoral.

The spokesperson declined to comment on the situation surrounding the Queen.

Per Sky News's Political Editor in the UK, Beth Rigby, Meghan Markle has not travelled to Balmoral with Prince Harry, Duke of Sussex.

Kate, the Duchess of Cambridge, has also not travelled to Scotland — she's stayed in Windsor while her three children stay in school.

 Typically the traditional Changing of the Guard ceremony takes place each morning outside Buckingham Palace — but with the Queen's health, a sign was placed outside saying it had been postponed.

The sign has since been removed.

News of the Queen's health has prompted people to gather outside Buckingham Palace in London.

It's currently mid-afternoon in England.

The Duke of Cambridge, who is second in line to the throne behind his father, Prince Charles, has just arrived at Aberdeen airport.

He was followed by Prince Edward and his wife, Countess Sophie of Wessex, and and Prince Andrew.

Newly sworn in UK Prime Minister Liz Truss was addressing the House of Commons when House Speaker Lindsay Hoyle interrupted debate about a new energy package to inform the House of Buckingham Palace's statement.

"I know I speak on behalf of the entire house when I say that we send our best wishes to her majesty the Queen and that she and the royal family are in our thoughts and prayers," he said.

In a post to his official social media channels, Australia's Governor-General David Hurley says his thoughts are with the Queen and the royal family.

"Following news from Buckingham Palace, and on behalf of all Australians, the Governor-General and Mrs Hurley’s thoughts and prayers are with Her Majesty The Queen and the Royal family at this time," he said.

Newly sworn-in British Prime Minister Liz Truss says she's "deeply concerned" by the news.

"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," she said in a statement.

Labour leader Keir Starmer said his thought were with the Queen and her family.

"Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon. My thoughts are with Her Majesty The Queen and her family at this time, and I join everyone across the United Kingdom in hoping for her recovery," he said.

Earlier today, representatives for the Queen had confirmed she had postponed a routine Privy Council to rest, on advice of her doctors.

The Privy Council usually meets once a month, and advises the Queen on the use of royal prerogative and statutory powers.

The council said the meeting would be rearranged.

Former British PM Tony Blair says he's "deeply concerned" to hear about the monarch's health.

Here's the latest from the ABC's Europe Correspondent Nick Dole in London.

The Duchess has chosen to stay at Windsor because her children, Prince George, Princess Charlotte and Prince Louis have started at school.

The royal family had released new photos of them visiting their new school overnight.

You can read more from royals expert Juliet Rieden below.

The monarch was photographed for the first time in two months on Wednesday (Australian time) after she appointed new British Prime Minister Liz Truss.

Ms Truss is the 15th Prime Minister the Queen has appointed since her reign began, and is the first Prime Minister the Queen had sworn in outside Buckingham Palace.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume.

Watch

Duration: 4 minutes 47 seconds4m 47s

After news of the Queen's ill health broke, members of the public have flocked to Balmoral Castle in Scotland's Aberdeen Shire.

If you're just catching up, the Queen is under medical supervision at her home in Balmoral in Scotland.

Members of the royal family, including Prince Charles and Camilla, the Duke of Cambridge and the Duke and Duchess of Sussex are on their way to Scotland to be at the Queen's bedside.

Prince Andrew, the Duke of York, is also said to be travelling to Balmoral.

In a statement, Buckingham Palace said her doctors recommended she remain under medical supervision.

"The Queen remains comfortable at Balmoral," the statement said.

Members of the royal family are travelling to Balmoral Castle to be with the Queen as her doctors say they are 'concerned for her health'.

Good evening, I'm Kate Ainsworth and I'll be bringing you the latest updates from Balmoral regarding the Queen's health.

The 96-year-old monarch has been suffering "episodic mobility problems" in the past few months, and was most recently seen swearing in new British Prime Minister Liz Truss.

Stay with us as we bring you the latest.