The Queen 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.
Prince Harry and Meghan are also making their way to Scotland, as are Prince Andrew and Prince Edward.
Prime Minister Liz Truss says “the whole country will be deeply concerned by the news”.
The Archbishop of Canterbury said he was sending his prayers and the first ministers of Scotland and Wales joined well-wishers in sending their thoughts.