LONDON – For the fourth time in five years, a prestigious multimillion-dollar prize offered to African leaders for good governance went unawarded Monday, renewing questions about the stringency of its rules, the paucity of contenders and the state of the continent's democracy.

The prize, named for Mo Ibrahim, a Sudan-born telecommunications billionaire, was designed to reward democratically elected African leaders who retire voluntarily at the conclusion of their mandated terms after displaying strong qualities of governance and leadership.

The prize is worth $5 million over the first 10 years, followed by stipends of at least $200,000 a year.

Since its creation seven years ago, it has been awarded three times, in 2007, 2008 and 2011. Former President Pedro Pires of Cape Verde was the most recent recipient. Former President Nelson Mandela of South Africa was given an honorary award.

The Mo Ibrahim Foundation said Monday that it had considered every African head of state or government who had retired in the past three years, but had decided not to award the prize this year.

The foundation did not elaborate on its reasons for withholding the award.

Salim Ahmed Salim, a former secretary-general of the Organization of African Unity and a former prime minister of Tanzania who is the chairman of a panel set up to consider potential winners, said in a statement that the award "honors former heads of state or government, who, during their mandate, have demonstrated excellence in leading their country, and by doing so, serve as role models for the next generation."

"After careful consideration," he added, "the prize committee has determined not to award the 2013 Prize for Excellence in Leadership."

The announcement came as the foundation made public its annual assessment of the state of African governance, which concluded that, while "overall governance continues to improve at the continental level," aspects of life in Africa like personal safety and the rule of law had "declined worryingly."

There is "a widening span in performance between the best and worst governed countries," the assessment said. Divergent trends within its findings, it said, "may sound a warning signal, with the new century seeing fewer regional conflicts but increased domestic social unrest."