A South African judge on Friday delivered a damning verdict against prosecutors' decision to drop more than 700 corruption charges against President Jacob Zuma, piling further pressure on the embattled leader.

"The decision... to discontinue the charges against Mr Zuma is irrational and should be reviewed," Pretoria High Court judge Aubrey Ledwaba said.

"Mr Zuma should face the charges as applied."

The charges, relating to a multibillion dollar arms deal, were dropped by the chief state prosecutor in 2009 — clearing the way for Zuma to be elected president later that year.

The prosecutor had justified dropping the charges by saying that recordings of tapped phone calls between senior officials in then-president and Zuma rival Thabo Mbeki's administration showed political interference in the case.

The main opposition party, the Democratic Alliance (DA), has since fought a long legal battle, claiming that the prosecutor's decision was wrong.

The recordings, which became known as the "spy tapes", were kept secret but finally released to the DA in 2014 after a five-year legal battle.

Zuma last month lost another major legal battle when the country's highest court found he violated the constitution over the use of public funds to upgrade his private residence.

READ: President Zuma must repay State, South African court orders

This led to an effort in parliament by the DA and other opposition parties to impeach him, but the ruling African National Congress (ANC) used its majority to easily defeat the motion.

Pressure on Zuma to be ousted or to resign has been growing with several veteran leaders of the party that brought Nelson Mandela to power in 1994 calling for him to step down.

Zuma is due retire from office in 2019 when his second term ends.

Editor's note: Story updated with judge's ruling.