Liz Truss has concluded her talks with world leaders and met the King as she prepares to say a final farewell to the Queen along with hundreds of dignitaries from across the globe.
She had been due to meet US President Joe Biden but the talks were cancelled on Saturday.
Instead, a “full bilateral meeting” has been scheduled for Wednesday when the pair are set to be in New York for the UN General Assembly.
Ms Truss also had an audience with Charles at Buckingham Palace on Sunday, ahead of his reception for visiting heads of state.
The weekend’s talks with world leaders were being framed by No 10 as chats to offer condolences over the Queen’s death, during which politics was likely to come up.
In particular, Ms Truss’s meeting with Irish premier Micheal Martin came against a backdrop of tensions over the future of the Northern Ireland Protocol.
Mr Martin said he had a “warm” meeting with the British PM but refrained from answering questions on the controversial post-Brexit treaty.
He left No 10 just under an hour after arriving on Sunday morning.
“I don’t think this is the time, if you don’t mind me saying so, to get into the detail about issues like the protocol,” he told the BBC.
“I’ve had a good telephone conversation last week with the British Prime Minister, had an initial warm meeting this morning where we discussed many issues in the context of the British-Irish relationship.
“But I do think the opportunity is there for us to reset relationships and to be conscious of what we achieved in previous years, the obstacles that were overcome then.
“And that gives us the strength to know that we have the capacity to overcome current obstacles, current issues that potentially could impede the relationship, and I believe we can overcome them.”
Ms Truss also met Canadian Prime Minister Justin Trudeau and Polish President Andrzej Duda at Downing Street on Sunday.
The PM kicked off her talks with world leaders on Saturday, speaking to counterparts from New Zealand and Australia.
The meetings were held at the Government’s Chevening country residence, rather than Chequers, which is said to be undergoing routine maintenance work after Boris Johnson’s exit.
At 8pm on Sunday, Ms Truss will observe a minute’s silence with the country to mourn the Queen’s death and reflect on her life and legacy.
People have been invited to mark the occasion privately at home, on their doorstep or street, or at community events and vigils.