Prime Minister Gordon Brown has hailed veteran US politician Ted Kennedy the "senator of senators" following his death at the age of 77.
Mr Kennedy, the brother of the late US president John F Kennedy, was diagnosed with a malignant brain tumour last year.
He had served alongside 10 presidents in a career spanning nearly 50 years in the Senate.
Mr Brown said he was proud to count Mr Kennedy as a friend, while former prime minister Tony Blair paid tribute to his role in the Northern Ireland peace process.
US President Barack Obama spoke of Mr Kennedy's "extraordinary life", adding: "In the United States Senate, I can think of no-one who engendered greater respect or affection from members of both sides of the aisle."
Mr Brown said Mr Kennedy, the senator for Massachusetts since 1962, would be mourned "in every continent".
He said: "He is admired around the world as the senator of senators. He led the world in championing children's education and health care, and believed that every single child should have the chance to realise their potential to the full.
"Even facing illness and death, he never stopped fighting for the causes which were his life's work. I am proud to have counted him as a friend and proud that the United Kingdom recognised his service earlier this year with the award of an honorary knighthood."
Mr Kennedy was awarded the knighthood for "services to British-American relationships and to Northern Ireland".
Tory leader David Cameron said Senator Kennedy had served the American people "in the spirit of a family whose name is synonymous with public service in the US". He added: "He fought for the causes he stood for, from civil rights to education and healthcare."
