Once again, there is no winner this year of a $5 million award for African leadership.

The Mo Ibrahim Foundation announced Thursday that no former African leader had met the "very high bar" for one of the world's richest prizes.

The Ibrahim Prize has been awarded just four times in the decade since it was established.

Candidates are African heads of state or government who have left office in the past three years, been democratically elected and served their constitutionally mandated term. They must also show "exceptional leadership."

The award honors role models on a continent where many leaders have stayed in power for decades.

"If there is no worthy candidate, the committee chooses not to award," the foundation's head of communications, Sophie Masipa, said in a statement.

Previous prize winners are presidents Hifikepunye Pohamba of Namibia in 2014, Pedro Pires of Cape Verde in 2011, Festus Mogae of Botswana in 2008 and Joaquim Chissano of Mozambique in 2007. Nelson Mandela was named the inaugural honorary laureate in 2007.

The founder of the prize is Mo Ibrahim, a British mobile phone magnate born in Sudan. His foundation also issues an annual index of the quality of governance in every country in Africa.

Mauritius, Cape Verde, Botswana, South Africa and Namibia top the current index with the highest average scores on safety and rule of law, human rights, sustainable economic opportunity and human development.