He also ruled that the NPA and Zuma must jointly pay the costs of the appeal as well as three legal counsel for the DA.
The Supreme Court of Appeal has upheld the ruling of the Gauteng High Court that the National Prosecuting Authority's decision to drop the corruption charges against President Jacob Zuma was irrational.
The judgment was handed down in Bloemfontein on Friday morning.
Leach said that it was clear that Deputy National Director of Public Prosecutions Willie Hofmeyr and Mpshe were "straining to find justification" for discontinuing the prosecution.
Leach had scathing words for Mpshe's decision, rubbishing his claims that former head of the Scorpions, Leonard McCarthy, manipulated the timing of Zuma's indictment. That would leave the decision in the hands of NPA boss Shaun Abrahams‚ who is seen as a Zuma ally.
Judge Eric Leach says that the NPA and President Zuma were correct to concede that the decision to withdraw criminal charges was not supported by law.
Either of the two may yet approach the Constitutional Court or they may decide to drop the the appeal route.
This means the NPA should immediately reinstate charges of fraud and corruption against Zuma.
Zuma's prosecution was discontinued by Mpshe based on the infamous "spy tapes".
In April a year ago, the North Gauteng High Court ordered that Zuma must face all the charges.
He added that, even if the court said Mpshe's decision had been irrational on further, substantive grounds, "then someone has to make a rational decision on our representations". The opposition Democratic Alliance asked the court to order holding an immediate trial.