An ambitious attempt to save Rome’s Colosseum from collapse is being undermined by the authorities’ cost-cutting decision to employ ordinary builders rather than specialists to perform the delicate overhaul, restoration experts have claimed.

The precarious state of the landmark, built in AD80, was underlined last year when three large chunks fell off. Discoloured by pollution from Rome’s constant traffic and rocked by vibrations from a nearby metro line, the amphitheatre was desperately in need of renovation – and the funding to pay for it.

Italian shoe magnate Diego Della Valle of Tod’s luxury leather goods stepped up in January this year to foot the €25m (£21.4m) restoration bill, in exchange for advertising rights at the site which attracts millions of tourists a year. The three-year facelift and restructuring is due to begin at the end of this month.

But this week, Carla Tomasi, president of the Rome-based Restorers Association of Italy, called on the government to stop the work or risk causing “irreparable damage” to amphitheatre.

According to the trade group, a government official charged with overseeing work on Rome’s archaeological sites two years ago changed contract bidding rules, in the process largely squeezing out art and archaeology restoration firms in favour of cheaper, large-scale building contracting firms that have less specialist knowledge.

In an open letter to Italy’s new culture minister Lorenzo Onarghi, the restorers group called on him to stop the bidding or “to prevent irreparable damage to Italy’s most celebrated monument and consequently damaging the image of our country”.

This is not the first time the ARI has slated the renovation plans. Last February it complained that an insufficient proportion of the €25m was being made available for reconditioning the Colosseum’s decorative structures, work for which its members were particularly qualified. And yesterday an ARI spokesman said work on the Colosseum was now about to begin in “inauspicious circumstances”.

The special treatment given the relatively lowly Ministry of Justice building has furthered angered the restorers. “Recently the government gave the contract for restoring the Ministry of Justice to specialist 19th-century restoration firms. Why then has the Flavian Amphitheatre been afforded less privileged treatment?” asked a spokesman.

But the director of the Colosseum, Rossella Rea, denied that specialists were not being used. “Restorers are always involved; they’re our collaborators and they’ll be called on if we need to clean travertine or marble,” she said.