Ashdown, who was one of the main advocates for international intervention during the Balkan conflicts of the early to mid-1990s, said the United Nations had a central role in maintaining world peace.

He was speaking at a service attended by Britain's Queen Elizabeth II and Prime Minister Tony Blair in Saint Paul's Cathedral, London, to commemorate the 60th anniversary of the UN's creation.

"We shall be lucky, I think, if the UN reaches 70 years without having confronted the real threat of nuclear, chemical or biological terrorism," Ashdown told the congregation.

"So the dangers may be new but the challenges of our time too can only be made by standing with the instruments of global governance.

"No-one can doubt the need for reform of the UN but no-one can doubt either the central role it has played in peace or the role it must play if we are to ensure peace for our time."

The service included a dedication and commitment to the UN's values in the future.

Ashdown, who led Britain's opposition Liberal Democrat party from 1988 to 1999, said upholding those values was vital in the "dangerous decades" ahead.

The high representative's role has among its functions the implementation of civilian aspects of the 1995 Dayton Peace Agreement, brokered to end the three-year Bosnian conflict, pitting the former Yugoslav republic's Croatians, Muslims and Serbs against each other.