Work is to begin in December on a two and a half-year, €25m (£19.5m) restoration of the Roman Colosseum, Italian officials said on Tuesday.

The mayor of Rome, Gianni Alemanno, said work would start in December and be completed by the middle of 2015.

Mariarosaria Barbera, the government official in charge of Rome’s archaeological monuments said a contract for the first stage of the restoration had already been awarded to a company that had offered to carry out the work for 26% less than the working estimate.

Scaffolding will obscure parts of Italy’s most visited monument for a planned 915 days. But, said Barbera, once the work was completed, underground sections of the Colosseum would be open to the public for the first time resulting in “an increase of 25% in the area that can be visited”.

“No monument, Roman or otherwise, was built to last forever,” said Barbera. But the work has been dogged by controversy. A consumer group says it will appeal against a court decision that cleared the way for work to start, over the awarding of the contract .

The group, known as Codacons, said it would ask for a ruling from Italy’s top administrative law tribunal on its claim that a deal for the funding of the work was unlawful because it had been handed, without competition, to the Tod’s shoe company.

The firm’s founder, Diego Della Valle, insisted he was not a sponsor, but a disinterested patron. His only condition, he said, had been “that there be no sort of commercial return”. A campaign would be mounted to publicise the restoration. But it would be carried out by a separate, non-profit organisation.

In January, it was announced that prosecutors in Rome and the Italian audit court had both launched inquiries into the funding arrangements for the project after Tod’s was reported to have obtained the right to use the image of the Colosseum for two years after its completion.

Barbera said: “No monument, Roman or otherwise, was built to last forever.” And the restoration — if it is not delayed again — is likely to increase pressure on Rome’s city council to divert traffic away from a building that was last given an overhaul 73 years ago.

Alemanno said a plan existed. But he added that it was linked to the digging of Rome’s third underground railway, which has been the subject of numerous delays.