King Charles led his siblings as they held a vigil over the Queen's body in Westminster Hall.
The new King, his brothers Andrew and Edward and sister Anne spent 15 minutes of quiet reflection standing guard over their late mother's coffin.
For the "Vigil of the Princes" the royals stood with their heads bowed at the four corners of Her Majesty's oak coffin.
It comes as the wait to see the Queen lying in state has blown out to a massive 22 hours.
At one point on Friday police shut entry to the queue when it stretched to more than 7km long.
On Friday evening it was reopened but officials were warning of a massive wait and to bring blankets to keep out the overnight chill.
Prince Andrew, the Duke of York, was allowed to wear military uniform for the vigil after Charles temporarily lifted the ban on him doing so.
The symbolic ceremony mirrored the vigil they held for the Queen while she laid in state in Edinburgh.
The Queen's eight grandchildren – Prince William, Prince Harry, Princess Beatrice, Princess Eugenie, Peter Phillips, Zara Tindall, Lady Louise Windsor and James, Viscount Severn – will hold a similar vigil on Saturday evening (Sunday morning NZT).
Harry will also be allowed to wear his military uniform, after previously being banned because he is no longer a working royal.
The Vigil of the Princes dates back to the death of King George V in 1936.
His four children – King Edward VIII (later the Duke of Windsor), Prince Albert (later King George VI), Prince Henry and Prince George – stood guard as he lay in state at Westminster Hall.
After the death of the Queen Mother in 2002, Charles, Andrew, Edward and Princess Margaret's son, the Earl of Snowdon, held a similar vigil in the same hall.
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