Edward Kennedy influenced a succession of US presidents
Gordon Brown and other UK politicians have paid tribute to US Senator Edward Kennedy, highlighting his role in the Northern Ireland peace process.
Mr Kennedy, who has died aged 77, was admired around the world and would be mourned everywhere, Mr Brown said.
Predecessor Tony Blair said Mr Kennedy had shown a "passionate commitment" to a lasting peace in Northern Ireland.
Tory leader David Cameron said Mr Kennedy was one of the most effective lawmakers in the history of the Senate.
Mr Kennedy, brother of John and Robert Kennedy, was a leading figure in US and international politics for more than 40 years.
He was diagnosed with a malignant brain tumour last year but remained active in politics and campaigned on behalf of Barack Obama during last year's US presidential election.
Mr Brown said Mr Kennedy was admired around the world for his efforts to help the under-privileged.
"Senator Edward Kennedy will be mourned not just in America but in every continent.
"He led the world in championing children's education and healthcare 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."
Mr Blair said Mr Kennedy had inspired devotion and respect around the world and highlighted the role he played in helping negotiate a lasting peace in Northern Ireland.
"He was a true public servant committed to the values of fairness, justice and opportunity.
"I saw his focus and determination first hand in Northern Ireland where his passionate commitment was matched with a practical understanding of what needed to be done to bring about peace and to sustain it."
Former Labour foreign secretary Lord Owen said Mr Kennedy had his flaws but he had made a key contribution to the Northern Ireland peace process.
He told the BBC that Mr Kennedy had initially been regarded by the UK as an apologist for the IRA but had changed his position and worked tirelessly "behind the scenes" to help broker peace.
"He saw, from our point of view, the necessity of dealing with this problem and he put his weight behind it," Lord Owen said.
"His influence on the peace process and his influence on successive US presidents, in particular on President Clinton, was absolutely crucial."
The Conservatives praised Senator Kennedy's commitment to public service and his courage in fighting his illness.
"He should be remembered as one of the most effective legislators in the history of the US Senate, a symbol of the strength and independence of the US legislature," said the party's leader David Cameron.
"He fought for the causes he stood for, from civil rights to education and healthcare."