Charles is confirmed as the next head of the 53-member alliance as a meeting in London draws to a close.
Prince Charles will succeed the Queen as the next head of the Commonwealth when she steps down.
Leaders of the 53-nation group are said to have made their choice on Friday during a retreat at Windsor Castle.
Speaking after the Commonwealth Heads of Government Meeting (CHOGM) Prime Minister Theresa thanked the Queen for her service and endorsed Charles to follow her in the role.
"His Royal Highness has been a proud supporter of the Commonwealth for more than four decades and has spoken passionately about the organisation's unique diversity, and it's fitting that one day he will continue the work of his mother, Her majesty the Queen," May said.
Some people, including the British opposition leader Jeremy Corbyn, had been calling for a rotating presidency for the informal alliance. However, the Queen, who turns 92 on Saturday, personally intervened in the debate, saying that it was her sincerest wish that Charles should succeed her.