JOHANNESBURG — South Africa's main opposition Democratic Alliance party on Sunday demanded a formal investigation into the breakdown of an ambulance carrying Nelson Mandela to hospital earlier this month.

"We have to be absolutely sure that the . . . military ambulance service poses no future risk to the health of former president Nelson Mandela," David Maynier, the shadow defense minister, said in a statement.

The military intensive care ambulance that rushed the 94-year-old anti-apartheid hero to hospital in the early hours of June 8 developed engine trouble, resulting in a 40-minute delay until a replacement ambulance arrived.

The military health service has "let the country down," Maynier said, adding that he would write to Defense Minister Nosiviwe Mapisa-Nqakula "requesting that a board of inquiry be convened to investigate the incident."

On Saturday, presidency spokesman Mac Maharaj said doctors were "satisfied" that Mandela suffered no harm during the wait for a replacement ambulance to take him from his Johannesburg home to a specialist heart clinic in Pretoria about 30 miles away.

The "fully equipped ICU (intensive care unit) ambulance" had a "full complement including intensive care specialists and ICU nurses," Maharaj told AFP.

The Nobel Peace Prize laureate is receiving treatment for a recurrent lung infection and his condition remains serious but stable, officials say.

Mandela, who will turn 95 on July 18, has been hospitalized four times since December, mostly for the pulmonary condition that has plagued him for years.