Ukraine's first lady Olena Zelenska has said Queen Elizabeth II "shared the values Ukraine stands for today".
After an audience with the Princess of Wales at Buckingham Palace, Ms Zelenska said the late Queen had "repeatedly" said words of support for Ukraine - which is fighting a war with Russia.
Ms Zelenska visited Westminster Hall to see the Queen lying in state before her meeting with Catherine.
World leaders have been arriving in London ahead of the Queen's funeral.
The lying-in-state ended at 06:30 BST, with hundreds of thousands of people queuing for hours to see the coffin.
Speaking to the BBC, Ms Zelenska said she was in the UK paying her respects to the Queen "on behalf of all Ukrainians".
"They know that the Queen shared the values that Ukraine stands for today: freedom, the right to one's own home, language, culture and country," she said. "We have repeatedly heard words of support from her."
"On Independence Day, she wished us better times in this extremely difficult year," she said. "In an address to the British Parliament, she spoke about supporting Ukraine.
"It is very important for us to know that such a world leader of an epoch, a leader with an impeccable reputation and morals, was with us."
Ms Zelenska's visit to the UK comes after her husband, President Volodymyr Zelensky, signed a book of condolence for the Queen at the British embassy in Kyiv.
The UK's ambassador in Kyiv, Melinda Simmons, said she was "deeply honoured" by the president's gesture "given all else that is happening in [Ukraine] at this time".
Ms Zelenska previously met Catherine and Prince William during an official visit to the UK in 2020 with her husband.
US President Joe Biden also saw the lying in state on Sunday and said the "world is better" because of Queen Elizabeth.
President Biden, accompanied by his wife Jill, said Britons "were fortunate to have had her for 70 years - we all were".
He also joined other world leaders in signing a book of condolences at Lancaster House.
Leaders from across the Commonwealth, of which the Queen served as head for the entirety of her reign, will also attend the funeral, including Canada's Justin Trudeau, New Zealand's Jacinda Ardern and Australia's Anthony Albanese.
Other world leaders to have accepted invitations to the funeral include Irish Taoiseach Micheal Martin and President Michael Higgins, German President Frank-Walter Steinmeier, Italian President Sergio Mattarella and French President Emmanuel Macron.
But Saudi Arabia's Crown Prince Mohammed Bin Salman is no longer expected to attend the funeral, a UK Foreign Office source says.