President Barack Obama has announced a new initiative to double access to electric power in sub-Saharan Africa, as part of his effort to build on the legacy of equality and opportunity forged by his hero Nelson Mandela.

Mr Obama and his family yesterday visited Robben Island, where the anti- apartheid leader spent 18 years in a tiny cell, including a stop at the lime quarry where Mr Mandela developed the lung problems that have put him in hospital.

The White House said Mr Obama's guide for the tour was South African politician Ahmed Kathrada, 83, who was also held at the prison for nearly two decades.

The President saw the prison courtyard where Mr Mandela planted grapevines that remain today, and where he and others in the dissident leadership would discuss politics, sneak notes to one another and hide writings.

Mr Obama wrote in the guest book in the courtyard: "On behalf of our family, we're deeply humbled to stand where men of such courage faced down injustice and refused to yield.

"The world is grateful for the heroes of Robben Island, who remind us that no shackles or cells can match the strength of the human spirit."

Mr Obama, First Lady Michelle Obama and daughters Malia and Sasha took in the expansive view of the quarry, a huge crater with views of the rusty guard tower from where Mr Mandela was watched.

Mr Obama visited retired archbishop Desmond Tutu before he gave a speech at the University of Cape Town to unveil the Power Africa initiative, which includes an initial $7 billion (£4.6bn) investment from the United States over the next five years. Companies including General Electric and Symbion Power are making a $9bn in commitments to provide power to millions of Africans.