Senior British royals were travelling to the Balmoral estate on Thursday to be with Queen Elizabeth II after doctors raised alarms about her health.
The queen's four children all went to the monarch's Scottish country residence after the queen, 96, was placed under the medical supervision.
Prince Charles, the heir to the throne, travelled with his wife Camilla, Duchess of Cornwall. His son Prince William went to Balmoral without his wife Kate, Duchess of Cambridge, who was looking after their children in their first week back at school.
Prince Harry, who is on a visit to Europe from his American home, cleared his diary to join his family in Scotland. There were conflicting reports about the whereabouts of his wife Meghan, Duchess of Sussex.
The news that the queen was unwell filtered through just as Ms Truss was making a major policy announcement on energy to MPs.
Amid worried glances in the House of Commons chamber, Speaker Lindsay Hoyle interrupted the debate to say the queen and royal family were "in our thoughts and prayers".
Sir Keir Starmer, the leader of the opposition, said he was "deeply worried" by the news from Buckingham Palace, while London Mayor Sadiq Khan wished the queen a swift recovery.
Former prime ministers Tony Blair and David Cameron also expressed their concern, while Archbishop of Canterbury Justin Welby said the nation's prayers were with the queen.
The statement issued by the palace at lunchtime said doctors were "concerned for Her Majesty’s health and have recommended she remain under medical supervision".
She "remains comfortable and at Balmoral," it said.
The queen, who marked 70 years on the throne this year, has cancelled a number of public appearances in recent months owing to what aides described as mobility problems.
She unusually remained at Balmoral on Tuesday to receive departing prime minister Boris Johnson and invite Ms Truss to form a government, a formality that would usually take place in London.