Burundi's top court has made a ruling aimed at ending the power vacuum created by the sudden death of President Pierre Nkurunziza on Monday.
The Constitutional Court said President-elect Evariste Ndayishimiye should be sworn in as soon as possible.
Legally, the Speaker of parliament, Pascal Nyabenda, should have become the interim leader.
But the cabinet decided to refer to the court and now Mr Ndayishimiye is due to become president two months early.
He was elected in May to replace President Nkurunziza, who had been in power for 15 years.
Burundi's top judges said that in the constitution the interim period under the speaker of parliament was designed as a time to hold fresh elections, but the president-elect's recent victory made this unnecessary.
Mr Nkurunziza had been able to run for a fourth term in last month's election but decided to retire and was to be known as the "supreme guide to patriotism".
The government said he died of a cardiac arrest after being taken ill on Saturday evening.
Will the new president mend fences?
By Anne Soy, BBC senior Africa correspondent
The ruling ends days of uncertainty about the succession in the small East African country where Burundians have been stunned by the death of their president.
But it will not ease all concerns - especially about Covid-19, which the government has downplayed for months. Rumours about the health of government officials and the first family abound.
Mr Nkurunziza insisted on holding May's election amidst the outbreak, and even expelled World Health Organization (WHO) representatives from the country.
The president-elect will have to decide if the status quo should continue - or chart a new path. He could do this by mending fences with the international community, including the WHO, to start dealing with the coronavirus pandemic, which the global health body has said is accelerating in Africa.
Did he have coronavirus?
There have been numerous reports that Mr Nkurunziza died after contracting coronavirus but these have not been confirmed.
SOS Médias Burundi, which is made up of exiled journalists, quoted both a medical source and another close to the government as saying that the late president had Covid-19.
The government has not responded to the reports.
Mr Nkurunziza appeared to downplay the pandemic several times.
In early March, before the country confirmed its first case, he said that "God will protect us" from coronavirus, then just a week before he died, he said "God has cleared [coronavirus] from Burundi's skies".
The country has recorded just over 80 cases of the virus and there has been one confirmed death.
Only gospel music allowed
Burundi is currently in a week-long period of national mourning following Mr Nkurunziza's death and the cabinet has suspended the playing of secular music in public places during this time.
Officials say gospel music is still allowed.
Mr Nkurunziza was a born-again Christian.