Dishevelled-looking backbencher Boris Johnson was the 15th MP to pledge his allegiance to the new king today.
The former Prime Minister, who was ousted from power following a mass walkout by fed-up Tory MPs in July, was one of the last visitors the late Queen received at Balmoral before her death.
Mr Johnson, MP for Uxbridge and South Ruislip, had characteristically ruffled hair as he walked into the House of Commons, which returned for the first time since the Queen's funeral this morning.
Holding the King James Bible, Mr Johnson said: "I swear by almighty God that I will be faithful and bear true allegiance to His Majesty King Charles, his heirs and successors, according to law, so help me God."
Hundreds of MPs are today pledging their allegiance to the new king following the passing of the Queen earlier this month.
MPs are sworn in after each General Election - and can only take their seat, speak in debates, vote and receive a salary once they have done it.
Today's pledge is optional for MPs, as they will have already pledged their allegiance to the heirs and successors of the Queen.
Mr Johnson was seen chatting to DUP MP Sammy Wilson and Transport Secretary Anne-Marie Trevelyan - who was first in line - as he waited.
Ms Trevelyan was followed in the line by Home Secretary Suella Braverman.
Commons Speaker Sir Lindsay Hoyle opened proceedings by thanking parliamentary staff, members of the armed forces and others for their efforts during the Queen's lying-in-state and funeral.
He told MPs: "I also want to put on record my gratitude to all those who helped ensure events of the last 10 days or so have been managed with such dignity and brilliance, albeit in the saddest of circumstances.
"That includes, of course, many outside this House, including our armed services, the police and countless others.
"I want to say a very special thank you especially to all those parliamentary staff, including colleagues who volunteered to take on roles beyond their day jobs who contributed and you ensured that Parliament was able to play its part safely, respectfully and with pride, and so I am extremely grateful for all that has been done."
MPs can swear the oath or affirm throughout Wednesday, with Sir Lindsay also advising: "I remind members that swearing-in is recorded by the television cameras, anything said or done by members may appear on television or may be picked up on the microphones."
Normal business in the Commons will resume tomorrow.
