Greece's prime minister has set up a task force and vowed to get to the bottom of allegations that a fraud based in his country bilked European Union agricultural funds out of 290 million euros ($343 million).

Kyriakos Mitsotakis admitted his government should have ensured the fraud could never have happened and said "the state's inadequacy is obvious. Clientelism (political patronage) cannot govern the way we conduct business".

He said his task force will not rest until it has rooted out all of those responsible.

Mitsotakis acknowledged that people could have been making fraudulent claims from Greece for EU agricultural subsidies for several years, and said his task force has been told to probe the allegations "immediately and exhaustively".

The prime minister, who has campaigned on rooting out corruption, later wrote on Facebook: "We failed."

Five senior government officials have already resigned over their alleged roles in the scandal, including Makis Voridis, a former far-right activist and prominent figure in the governing New Democracy party who served recently as migration minister and who was agriculture minister at the time the alleged fraud began.

Another former agriculture minister in Mitsotakis's government, Lefteris Avgenakis, has also resigned, after being named by EU investigators, who said the two ministers knew about the fraudulent claims for subsidies but did nothing to stop them.

Investigators from the Luxembourg-based European Public Prosecutor's Office, or EPPO, who launched an investigation into the alleged fraud in 2021, informed the Greek government last week about their findings via a 3,000-page report that claimed the two government ministers were involved, and that 15 other lawmakers and several regional officials were also at the heart of the scam that involved hundreds of people who had been declared to be the owners of pasture and grazing land, despite not being farmers.

Voridis said he stood down to focus on "defending my innocence".

Investigators believe the fraud began in 2017 and lasted for around five years.

Mitsotakis said his task force will conduct its investigation "in line with internal and European legislation" and that the Greek agency that had distributed EU subsidies, the OPEKEPE, will be disbanded. He added that because it "didn't manage to do its work, the state will do it centrally".

Allegations about wrongdoing at the OPEKEPE prompted the European Commission to impose a 415-million-euro fine on Greece on June 17.

Mitsotakis's task force will also look into claims that officials who had concerns about the OPEKEPE were discredited, demoted, or moved from their jobs.

The Politico news website described the scandal as "one of the biggest farming frauds of recent years" and noted that it will have been both politically damaging and embarrassing to Mitsotakis.