More than 470,000 people have signed a petition to pressure Mr Orban’s government into joining the European Public Prosecutor’s Office (EPPO), the EU’s new anti-corruption watchdog for EU funding.
The EU has allocated €25bn (£21.3bn) to Hungary in the 2014-2020 budget period and critics have warned funds are often spent on overpriced or unnecessary projects that benefit allies of the prime minister.
Mr Orban is known for his anti-EU and anti-immigration campaigns, and his accusations that the EU is increasing its powers to the detriment of member states.
Independent MP Akos Hadhazy launched a petition in September with the hope of gathering 1 million signatures to pressure Hungary’s government into joining the EPPO, which is expected to begin investigations at the end of 2020.
Mr Hadhazy and other critics say the country’s chief prosecutor, Peter Polt, has failed to investigate many cases of suspected corruption, some of which allegedly concern Mr Orban’s family.
“There are two bastions that keep this populist regime in power – one is the chief prosecutor and the other is public media and government propaganda,” Mr Hadhazy said last week while helping activists gather signatures.
Mr Polt “has been successfully attempting to whitewash the corruption affairs involving the Hungarian government ... and the most prominent politicians,” he added.
Foreign minister Peter Szijjarto defended Mr Polt on Monday, calling criticism of the former Fidesz parliamentary candidate “a coordinated series of political attacks”.
In a recent survey on corruption in the public sector by Transparency International, Hungary was ranked 26th out of 28 EU countries.
Although more than 470,000 Hungarians have signed Mr Hadhazy’s petition so far, it is not legally binding.
The government has rejected joining the EPPO, citing national sovereignty concerns and even misgivings about its quality.
The list of renovations and constructions partly funded by the EU in Budapest includes €572m (£487m) given for the city’s fourth subway line, €37.5m (£32m) for the Franz Liszt Academy of Music, and €10.5m (£8.9m) for construction at City Park’s ice-skating rink, the largest in Europe.
Hungary may have to repay funds to the EU if projects are found to not comply with EU rules, but observers say there is plenty left over for government allies.
“In the past years, it has become clear that there are a few businesspeople close to the government whose companies nearly always win the large public contracts,” said Katalin Erdelyi, an award-winning journalist for investigative news site atlatszo.hu.
“The investments are frequently overpriced, costing much more than initial estimates.”
Ms Erdelyi said there are projects that appear to be built with the only purpose of using as much of the EU’s funds as possible.
One of those projects is a bridge and “bicycle adventure park” that runs through the town of Hatvan and is used by few bikers due to safety concerns over its construction.
“It would be very good if Hungary joined the European Prosecutor’s Office,” Ms Erdelyi said.
“It would be much easier to uncover the deals suspected of corruption and confirm whether or not there was any misuse of European Union subsidies.”
Mr Orban’s Fidesz party is also currently involved in a dispute with the European People’s Party, which is the largest political group in the European parliament.
Fidesz could be suspended or ejected from the group when the EPP meets for its political assembly on 20 March over criticism of Mr Orban’s commitment to democratic principles.
The Hungarian leader has repeatedly said he would prefer to stay in the centre-right alliance, in order to transform it into an anti-immigration force.
The conflict between Fidesz and the EPP has been exacerbated by EU proceedings launched last year over concerns about the rule of law in Hungary since Mr Orban’s return to power in 2010.
Istvan Hegedus, a former Fidesz MP and current chairman of the Hungarian Europe Society, says Mr Orban is unlikely to take Hungary out of the EU.
Despite his clashes with the organisation, Mr Orban still relies on EU money.
“The funds, in part, generated non-sustainable growth, but if these financial resources were to end, the Hungarian economy would be in a much more difficult situation,” Mr Hegedus said, naming EU farm subsidies as one of those vital for Hungary.
“He will have to think over a thousand times whether he’ll give up these EU subsidies.”
Agencies contributed to this report
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