Following the accession ceremony on Saturday morning, the Speaker and selected MPs took an oath of allegiance to the new King in the House of Commons.
This is a rare Saturday sitting, with condolences following the Queen’s death also set to be given. The session is set to last until about 10pm.
It will end with a “formal humble address” to the King, “expressing the deep sympathy of the House” following his mother’s death.
Every MP will have the option of taking an oath to the King when the House returns, but they are not obliged to.
Commons Speaker Sir Lindsay Hoyle will determine the timetable in the House for the following days, but it is expected to be significantly reduced until after the Queen’s state funeral.
This means new laws cannot be passed until Parliament returns, though it could be recalled for urgent matters.
Following King Charles officially being declared the nation’s new monarch at St James’s Palace, he has arrived back at Buckingham Palace for meetings on Saturdday afternoon.
Excited crowds were there to greet him following his historic accession ceremony, and he could be seen smiling and waving back at well-wishers.
The monarch will also meet the Archbishop of Canterbury at Buckingham Palace, followed by a meeting with Prime Minister Liz Truss and cabinet members later and then leaders of the opposition parties at 3pm GMT.
At 4pm GMT, the King will meet the Dean of Westminster at Buckingham Palace.
Separately, the Royal Family is expected to confirm details of arrangements for the period of mourning, including possibly the date of the Queen’s funeral.