Volodymyr Zelenskyy has said he is "exchangeable" for Ukrainian membership of NATO, indicating he would be prepared to step down as president if his country was allowed to join the military alliance.
He also suggested he was ready to sign a minerals deal with the US that was put on hold after his heated meeting with Donald Trump in the Oval Office on Friday.
Mr Zelenskyy was speaking to reporters on Sunday evening after earlier talks with other world leaders at a London summit hosted by Sir Keir Starmer, who suggested a coalition of European allies could step up to defend Ukraine and "guarantee the peace".
Latest updates on Ukraine's future
Asked by Sky News' lead world news presenter Yalda Hakim about his previous comments that he could resign in the event that his country becomes a NATO member, he said: "I am exchangeable for NATO."
The Ukraine president added: "I have said that I am exchanging for NATO membership, then it means I have fulfilled my mission. NATO means I have fulfilled my mission."
But, he also said: "To change me, it will not be easy because it is not enough to simply hold elections. You would need to prevent me from participating. And it will be a bit more difficult."
As the three-year full-scale war between Russia and Ukraine rages on, Mr Zelenskyy has also rebuffed US officials calling for him to resign.
His comments at Stansted Airport come after Republicans including senator Lindsay Graham and speaker Mike Johnson suggested he should step down from his position following the fiery White House exchange with President Trump.
Mr Zelenskyy spoke to journalists shortly before leaving the UK on Sunday evening.
In response to a question from Yalda Hakim, Mr Zelenskyy said: "I can give [Lindsay Graham] citizenship of Ukraine and he will become a citizen of our country.
"And then his voice will start to gain weight, and I will hear him as a citizen of Ukraine on the topic of who must be the president."
"The president of Ukraine will have to be chosen not in Lindsay Graham's home but in Ukraine," he added.
Mr Zelenskyy confirmed there has been communication between Ukraine and the Trump administration since the clash on Friday, but "not on my level".
Mr Graham - a Republican senator and close Trump ally - labelled the meeting a "complete, utter disaster" at a press briefing on Friday.
Asked whether Mr Zelenskyy should step aside, he said: "He either needs to resign or send somebody over that we can do business with, or he needs to change."
After the London summit on Sunday, the prime minister unveiled a four-step plan discussed by leaders, including a pledge to "develop a coalition of the willing" to defend a deal in Ukraine and guarantee a peace settlement.
Sir Keir said Europe "must do the heavy lifting", and the UK "is prepared to back this with boots on the ground and planes in the air, together with others".
He also announced a new deal which will allow Ukraine to use £1.6bn of UK export finance to buy 5,000 more air defence missiles.
Zelenskyy's meeting with the King
The Ukrainian president's 90-minute sit down with the media came shortly after he wrapped up a tough week with a visit to see the King at Sandringham.
Mr Zelenskyy flew to Norfolk in a helicopter on Sunday afternoon after attending the security summit in central London.
People - some holding Ukraine flags - gathered outside Sandringham to try to witness his arrival. A military helicopter could be seen flying low before descending over the estate.
Pictures released after the meeting show Charles and Mr Zelenskyy shaking hands by the entrance to the royal home. The pair chatted briefly before heading inside, where they posed for more photographs.
Their meeting lasted just under an hour, Sky News understands.
A difficult week for Ukrainian president
Mr Zelenskyy's difficult few days began on Friday, when his day at the White House with Mr Trump was cut short after their meeting in the Oval Office descended into a shouting match.
The pair had been due to sign a minerals deal and hold a joint press conference - but both events were called off after their tense exchange, fuelled in part by comments from vice president JD Vance.
Read more:
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As White House officials scrapped the day's schedule, Mr Trump described the meeting as "very meaningful" in a post on his Truth Social platform.
He wrote: "I have determined that President Zelenskyy is not ready for Peace if America is involved, because he feels our involvement gives him a big advantage in negotiations."
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Meanwhile, in an interview on Fox News, the Ukrainian president said the row was "not good for both sides" and refused to apologise. Mr Zelenskyy then jetted to the UK to meet Sir Keir and other world leaders.
(c) Sky News 2025: Volodymyr Zelenskyy tells Sky News 'I am exchangeable for NATO membership