South Africa’s president, Jacob Zuma, has suffered another blow to his already tarnished reputation after a court ruled that he should face almost 800 corruption charges that were dropped in 2009.

The high court in Pretoria deemed the decision to drop the charges by the national prosecutor at the time, Mokotedi Mpshe, as “irrational”.

“The decision … to discontinue the charges against Mr Zuma is irrational and should be reviewed,” said the Pretoria high court judge Aubrey Ledwaba. “Mr Zuma should face the charges as applied.”

Mpshe’s decision to drop the 783 charges, which date to 1989 and are linked to a multibillion-dollar arms deal, cleared the way for Zuma to be elected president that year. Mpshe had argued that recordings of tapped phone calls between senior officials in then-president Thabo Mbeki’s administration showed political interference in the case.

However, Ledwaba said on Friday that Mpshe’s thinking and behaviour was irrational, especially his failure to disclose his decision to prosecutors until the moment he announced it at a news conference.

“If the decision had been rational and above board, why the secrecy?” Ledwaba said. “Mr Zuma should face the charges as applied in the indictment.”

It was not immediately clear whether Zuma would appeal against the unanimous ruling. The judgment does not automatically reinstate the charges against Zuma, as that decision would be up to the national prosecuting authority. Friday’s high court decision follows a long legal battle by the main opposition party, the Democratic Alliance, which argued that the 2009 decision was wrong.

“Jacob Zuma is not fit to be the president of this country,” the Democratic Alliance leader, Mmusi Maimane, told reporters. “The decision that they took was irrational and we still maintain that Jacob Zuma is corrupt. Jacob Zuma must face the full might of the law. He has already violated the constitution. There is no debate about that.”

Zuma’s office said: “These charges were formally withdrawn by the high court in Pietermaritzburg during April 2009 and as such there is no pending litigation before the court against President Zuma. As a party to the proceedings, the president has noted the decision of the court and will give consideration to the judgment and its consequences and the remedies available in terms of our law.”

It is important to note that the Court did not deal with the merits of any allegations against President #Zuma — ANC (@MYANC) April 29, 2016

The constitutional court, South Africa’s highest court, ruled last month that Zuma had ignored an order to repay state funds spent on a lavish upgrade of his private home. The “security upgrades”, which cost taxpayers $24m (£16.4m), included a swimming pool, chicken run, cattle enclosure and an amphitheatre.

The decision precipitated an impeachment attempt by opposition MPs in early April. Zuma survived after the African National Congress party gave him its backing. An ANC spokesman said Friday’s decision would not hurt the party’s prospects in local elections in August.

Speaking on the eNCA television news channel, Zizi Kodwa said the ANC was not just about one leader but was a “collective”.

But the president is facing a growing chorus for him to step down amid a slew of scandals. Before the ruling on corruption charges and besides the home renovations imbroglio, Zuma has faced criticism involving the Guptas, a family of wealthy industrialists with close ties to the president.

The Guptas have been accused of exploiting their relationship with the president to choose pliable candidates for top cabinet and business jobs in a scandal that came to light after the country’s respected finance minister, Nhlanhla Nene, was abruptly sacked in December.