Rajapaksa said the restoration of full executive powers was necessary to implement his agenda to make the country of 21 million economically and militarily secure.

However, analysts say any attempt by Gotabaya Rajapaksa to push for changes that will strengthen presidential power at the expense of those of the Prime Minister may trigger sibling rivalry.

The brothers are best known for crushing the Tamil Tiger rebels fighting for a separate homeland for minority Tamils during the elder Rajapaksa's presidency in 2009, ending a 25-year civil war.

"We will ensure [Sri Lanka] will not stand disappointed during our tenure," Mahinda said on Twitter.

Currently, significant power is bestowed on Parliament and the prime minister after a previous government led by the opposition had amended the constitution and set up independent commissions to oversee the police and the judiciary among other arms of the government.

"We have seen in the past when governments have had a two-thirds majority it does not have to worry about checks and balances," said historian and political scientist Jayadeva Uyangoda.

"In fact they have untrammelled power, that is what we have observed in both India and Sri Lanka. In the past in Sri Lanka the governments which had this level of power in 1970 and thereafter in 1977 generated a lot of social discontent."

The country's economy contracted by 1.6 per cent in the first quarter of 2020, as COVID-19 restrictions battered the recovery from last year's Easter Sunday bomb attacks that killed 269 people and paralysed the tourism industry.

On a congratulatory phone call from Prime Minister Narendra Modi of India, which is keen to check Chinese influence on its southern neighbour, Mahinda Rajapaksa vowed to deepen ties between the two countries.

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"With the strong support of the people of Sri Lanka, I look forward to working with you closely to further enhance the long-standing cooperation between our two countries," he told Modi. "Sri Lanka and India are friends and relations."

Gotabaya was elected president last November after projecting himself as the only leader who could secure the country after the terrorist attacks in April. Since being elected, he has said he had to function under many restrictions because of the constitutional changes.

However, Mahinda is unlikely to cede any of his powers and shrink his influence as he works on promoting his son, Namal, as heir. Namal and three other members of the Rajapaksa family contested the election and are likely to control key functions in the new administration.

The landslide victory also raises fears of weakening government institutions such as independent commissions for elections, police and the public service.

More than 70 per cent of more than 16 million eligible voters cast ballots following health guidelines to prevent the spread of the coronavirus.

The election was originally scheduled for April, but it was twice postponed because of the COVID-19 pandemic. The island nation has largely contained the spread of the virus with 2839 confirmed cases, including 11 deaths.

Reuters, AP