Opponents of Jair Bolsonaro have seized on an unfolding scandal in Brazil over the procurement of coronavirus vaccines to turn up the political heat on the rightwing president.

Allegations of corruption in a deal to buy 20m doses of the Indian-made Covaxin jab have stoked criticism of the government’s handling of the pandemic, which has left more than half a million people dead in Latin America’s most populous country.

The controversy also comes at a politically sensitive time for Bolsonaro, who is gearing up for a re-election battle next year.

In opinion polls he trails Luiz Inácio Lula da Silva, the former leftist president who is expected to run in 2022 after having criminal convictions for corruption set aside earlier this year.

The Covaxin affair centres on claims that a civil servant in the health department was pressured by a senior government official to sign off an order for the shots, produced by Bharat Biotech, despite supposed irregularities in the paperwork.

The price agreed for the vaccines, $15 a dose, is more expensive than others bought by Brazil.

There are also questions about the role played by a company that acted as intermediary and why the contract was signed ahead of deals with Pfizer and Johnson & Johnson’s pharma division Janssen, even though Covaxin had not yet completed late-stage clinical trials.

Luis Miranda, a federal lawmaker and brother of the whistleblower who first flagged the Covaxin contract issues, has said they personally raised the concerns in March with Bolsonaro.

Bolsonaro’s administration has denied any wrongdoing with the R$1.6bn ($320m) agreement in February to acquire the jabs. The president has insisted that no money had been paid, since none of the vials have been shipped.

“There is no way for me to know what happens in the ministries, I trust the minister. We did nothing wrong,” he told supporters this week.

Even so, the health ministry on Tuesday moved to temporarily suspend the Covaxin contract following a recommendation from the federal comptroller.

The revelations have provided impetus to adversaries of the former army captain, who has come under fire at home and abroad for denying the gravity of Covid-19.

Following testimony last week by Miranda and his brother before a congressional inquiry established to investigate how the administration has dealt with the Covid-19 crisis, three senators have accused the president of not acting on the reports, lodging a criminal complaint with the supreme court.

“He did not take any action after being notified of the existence of a gigantic corruption scheme existing in the ministry of health. Prevarication is a crime,” said senator Randolfe Rodrigues, the commission’s deputy chair.

The federal prosecutor’s office is separately investigating the Covaxin negotiations.

Opposition lawmakers are set to submit on Wednesday a motion to begin impeachment proceedings against Bolsonaro, aiming to unite arguments from more than 100 separate requests already filed against him.

“It is evident that this government is not only genocidal but also corrupt,” said deputy Elvino Bohn Gass of the leftwing Workers’ party.

The presidency and health ministry did not respond to requests for comment. Bharat has previously denied wrongdoing in the supply of Covaxin.

For now, analysts say there is little chance Bolsonaro will be removed from office, since any impeachment request must be approved by the speaker of the lower house in Congress, who so far has signalled no intention to proceed.

Despite declining approval ratings, the president still commands the support of more than 20 per cent of voters, according to polls.

But the developments underline the pressure building on Bolsonaro, whose reputation has taken a battering during the congressional inquiry which has cast a poor light on Brasília’s response to the public health crisis.

During televised sessions, witnesses have spoken of the president’s support for discredited remedies, such as hydroxychloroquine, and allegations the government ignored dozens of emails about vaccine supply from Pfizer.

Just 12 per cent of Brazil’s population of 210m has so far been fully immunised against the virus.

Jimena Blanco, head of Americas research at Verisk Maplecroft, said the vaccines scandal had further weakened Bolsonaro’s congressional support, which would make it harder to push through important economic reforms.

“We can’t disregard these revelations resulting in an investigation by the supreme court for an alleged crime committed while in office,” she said. “The process for that would be much faster than a political impeachment.”

That decision lies in the hands of prosecutor-general Augusto Aras, a Bolsonaro ally, who must assess the senators’ complaint of “prevarication”. The offence is defined as a public official’s failure to fulfil a duty for reasons of personal interest, carrying a penalty of up to a year in prison and a fine.

Davi Tangerino, a professor of criminal law at the Getúlio Vargas Foundation, said Aras could request the case be closed for lack of evidence, expand the investigation or pursue a complaint against the president.

“If the prosecutor-general chooses to archive the case, there is nothing the judiciary can do about it,” he added.