This year's prize was given to Sampaio and to an ophthalmologist from Namibia, Helena Ndume.


The prize was created in a vote last year by the UN General Assembly, to pay homage to figures who have dedicated themselves to promoting the ideals of the UN.


It also aims to pay tribute to the efforts of Mandela himself for "reconciliation, political transition and social transformation", the assembly said in a statement.


Sampaio, 75, was president of Portugal for 10 years from 1996, before which he served as mayor of Lisbon. From 2006 to 2012 he was the UN special envoy in the fight against tuberculosis.


In recent years he has focussed on an initiative to supply emergency support for Syrian students so they can continue their studies, disrupted by the war in that country.


In a statement, the UN described Sampaio as a "great supporter of Portuguese democracy" since his time as a student activist during the dictatorship and, after the 1974 Revolution, as a politician.


Ndume, meanwhile, is cited for her work over two decades creating ophthalmology centres across Namibia, which have helped some 30,000 people to recover their sight and which offer free check-ups and treatment for problems such as cataracts.


The prizes are to be awarded at a ceremony at UN headquarters on 24 July, as part of ceremonies to mark the life and work of Mandela, who died in December 2013.