The Philippine president Rodrigo Duterte won a convincing victory in midterm elections that were seen as a referendum on his controversial administration, official results show.

Announcing the tally of results that was delayed by glitches in the automated counting machines, election officials said the winners included nine candidates backed by Mr Duterte for the 12 Senate seats up for grabs. The other three went to independents.

Only half the upper house’s 24 seats were being contested at this election, alongside positions for a total of 18,000 mayors, governors and local officials.

But the results give Mr Duterte an effective majority in the Senate, and tighten his grip on a country where he faces allegations of authoritarianism, sexism and widespread human rights violations.

The successful Senate candidates include Mr Duterte’s former national police chief, Ronald dela Rosa, who enforced the president's crackdown on illegal drugs in a campaign that left thousands of suspects dead and drew international condemnation.

Mr Duterte's three children also won races for mayor, vice mayor and a congressional seat representing their southern home region of Davao city. It sets up the possibility of a family succession when the president reaches the end of his single, six-year term as stipulated by the Philippine constitution.

With the president also in control of the lower House of Representatives, the Senate has so far been the most important check on his power, moving to block a number of more controversial measures.

Analysts expect Mr Duterte to use his control of both chambers to push ahead with plans that include massive infrastructure spending, the return of the death penalty and potentially even re-drafting the constitution to set up a federal style of government.

When asked about concerns that the new Senate would be beholden to Mr Duterte, Senator-elect Bong Go, a longtime aide to the president, said: “Do I look like a rubber stamp?”

He nonetheless stressed he would back the president's war against criminality, corruption and illegal drugs and would support a bill to reimpose the death penalty for heinous crimes and drug trafficking. Mr Go said Duterte has not given any illegal orders to him or anyone he supervised.

During the campaign, Mr Go said he felt Filipinos were not ready yet to support a shift to a federal form of government partly because of a lack of adequate information campaign about its benefits. "It's a longshot and it'll be difficult for us to work for the approval of federalism at this time," he said.

Mr dela Rosa, in a separate news conference, said: "My no 1 agenda is the reimposition of the death penalty for drug trafficking," adding the drug menace remains troubling despite Mr Duterte's crackdown.

As much as they are a boon to Mr Duterte, the midterm results are a significant blow to the opposition, which campaigned on a liberal message but was beset by lack of funding and other campaign issues. The opposition failed to take a single one of the contested Senate seats, meaning they now hold just four in total.

Election officials said nearly 75 per cent of more than 63 million registered Filipinos cast their votes in a strong turnout at the 13 May elections.