It is meant to be the highlight of any trip to Greece: climbing the "holy rock" in Athens to see the marvels of the ancient Acropolis. But visiting the place that Le Corbusier, the pioneer of architectural modernism, called the most "ruthlessly flawless" monument in the world is not what it used to be.

Parts of the Acropolis have been dismantled. Other areas are shrouded in scaffolding and overshadowed by a crane.

Thirty years after Greek conservationists launched the biggest restoration project in modern history, the works have become dogged by controversy, and the government in Athens has now revealed that at least 20 more years - and up to €70m (£47m) - will be needed to finish the project.

The restoration is causing political ructions in Greece, not least because nobody knows where the money will come from.

"I don't deny that there have been problems but this is the biggest renovation ever conducted on a monument in the world," the deputy culture minister, Petros Tatoulis, told the Guardian. After this, for generations to come, work will never be needed on the Acropolis again, he said.

"Technical progress is such that it will be able to withstand the most extreme weather conditions - earthquakes, you name it."

That was little consolation to the tourists visiting the Acropolis this week.

"Where is the temple of Athena Nike?" asked one woman as she looked at the empty space where the icon to Ionic art has stood since 500BC. "I can't believe they've actually dismantled it."

"You can't say it's very pretty," said Mark Rowlings, a photographer with the RAF, after making the arduous hike up the hill yesterday. "The crane and scaffolding really get in the way. It's not that it's not tremendous. It's just that I know I'll have to come back to see the real thing."

On a site that had been a prehistoric royal sanctuary, the classical monuments of the Acropolis were built by Pericles, the Greek statesman, who wanted them to be a lasting monument to the civilisation.

Because the Acropolis - which means the highest point of the town - is regarded as the ultimate monument of antiquity, the process of restoration has been mired in bureaucracy and debate.

Lengthy discussions take place over the tiniest interventions on stones that, over a span of 25 centuries, have survived fire, bombshells, looting and earthquakes.

Nikos Toganides, the engineer heading the preservation works on the Parthenon, said: "You never know what you're going to find. Things crop up. It's unpredictable work and that makes the whole process a lot slower."

But, say architects, it is the row over funding that is slowing them down most.

"If they want the scaffolding to go and us to get out of here then they have to resolve this problem," Mr Toganides said. He added that he had not been paid for the past three months.

"Nearly all of the team works on contract. Psychologically, it disrupts the flow of work and makes things a lot harder if people don't know when the next lot of money is coming."

This month, two of the Parthenon's most experienced marble carvers will retire.

But conservationists insist it is vital to keep the works going uninterrupted.

Restorers are working round the clock to replace with non-corrosive titanium rods the thousands of rusting iron clamps that were installed in an earlier misguided attempt to strengthen the temples.

"If it wasn't for the press here constantly raising the case, I'm sure the monies we requested last year and have only just received would not have been forthcoming," said Tasos Tanousas, an architect who has been at the Acropolis since the latest repair project began in 1975.

"There has been quite a bit of official indifference since the [Athens] Olympic Games."

The restoration has cost €35m so far, with at least half being funded by the European Union.

Last month, Greece's ruling conservatives agreed to inject a further €12.5m into the restoration works, but only after a period of intense haggling and once the EU had offered to contribute half the amount.

The furore has got many experts wondering whether the time has come to allow private sponsorship of the work. Until now, the idea has been rejected on the grounds that only Greek officialdom should deal with a monument of such grave importance to the country.

Mr Tatoulis, the deputy culture minister, has not ruled out private funds one day being poured into the "holy hill".

Recently, he said, the electronics group Phillips had used the Parthenon as a backdrop in an advertisement, but only after the government had agreed that as a one-off it would promote tourism.

"It can't be excluded, but if there was ever to be private sponsoring it would have to be very tightly regulated. "We couldn't have big firms defiling the Acropolis," Mr Tatoulis said.