Ghana's President John Atta Mills, who was suffering from throat cancer, has died in hospital in the capital, Accra.A statement from his office said the 68-year-old died a few hours after being taken ill, but did not give details."It is with a heavy heart... that we announce the sudden and untimely death of the President of the Republic of Ghana," the statement said.Mr Atta Mills had ruled the West African country since 2009.The BBC's Sammy Darko, who is at the military hospital in Accra, says Mr Atta Mills' voice has been degenerating in the last few months.'Rumours'According to a presidential aide, the leader had complained of pains on Monday evening and died on Tuesday afternoon.He had recently returned to Ghana after visiting the US for medical checks, the news agency says.Vice-President John Dramani Mahama will be sworn in as president at 2000 GMT, in line with Ghana's constitution.Journalist Elizabeth Ohene, a former BBC editor and member of Ghana's former government, said she had not believed the news at first, as there had been many false reports about his death."For the past three or four years there's been news he's been unwell and rumours of his death - twice - and he appeared with grim humour to say they were exaggerated, insisting he was well," she told the BBC's Focus on Africa programme.Mr Atta Mills served as vice-president to former Ghanaian military ruler Jerry Rawlings between 1997 and January 2001.He came to power after narrowly winning against a candidate from the then governing New Patriotic Party, Nana Akufo-Addo, in polls in December 2008.His predecessor, John Kufuor, stepped down after having served the maximum permitted two four-year terms.It was Mr Atta Mills' third attempt to win the presidency, after defeat in 2000 and 2004 to Mr Kufuor.Under Mr Atta Mills' leadership, Ghana joined the ranks of the world's large-scale oil producers. He was to run for a second term in December.BBC reporter said the NPP's presidential candidate, Nana Akufo-Addo, has cut short his election campaign in the west of Ghana.He is expected to express his formal condolences when he reaches the nearest town.