Her Majesty Queen Elizabeth II is under medical supervision at Balmoral Castle in Scotland, a statement from Buckingham Palace says.
Her doctors are concerned for her health, following further evaluation this morning, it says.
But the palace adds the 96-year-old monarch “remains comfortable”.
Prince Charles, Camilla and Princess Anne are at Balmoral and Prince William is travelling there, UK media reports.
Prime Minister Liz Truss says “the whole country will be deeply concerned by the news”.
Meanwhile, Sri Lankan President Ranil Wickremesinghe has also expressed concern on reports of the ill-health of Her Majesty Queen Elizabeth II and wished her a speedy recovery.