Giant cranes move into place tomorrow at the Uffizi Gallery in Florence to begin work on a €30 million (£20 million) plan to increase exhibition space at one of the world’s foremost treasure houses of art.

The extension scheme, which will last between four and five years, envisages demolishing the remains of a medieval church where Dante and Boccaccio are said to have preached.

The project will increase space at the Uffizi from 8,100sq m (87,000sq ft) to 12,900sq m. Italian culture ministry officials insist that the gallery can no longer cope with its two million visitors a year.

New space is being carved out of the first floor of the building, formerly occupied by the Florence city archives, as well as below the two wings. The roof is to be replaced by a glass superstructure, allowing daylight into overcrowded and badly lit exhibition rooms.

The project involves new lifts, stairs, cafeterias, visitor centres and lavatories. Conservationists say that the works will mean the demolition not only of “Dante’s church” but also of a vault by Giorgio Vasari, who designed the Uffizi for the Medici dynasty, as well as potentially risky excavations beneath the building.

Riccardo Francovich, Professor of Medieval Archeology at the University of Siena, a consultant to the project, said that concerns over “these sacrosanct stones” had been brushed aside. He said that the foundations of the Uffizi and the San Pier Scheraggio church formed the most important archeological site in Florence, containing precious evidence of the city’s origins in the fifth century.

Conservationists also protest that the contract was awarded to a Bologna-based consortium without being put out to tender. The project — first mooted more than 40 years ago — was drawn up by the government of Silvio Berlusconi three years ago and has been given the go-ahead by the present Government of Romano Prodi. Massimo Nuci, the engineer who is spokesman for the project, said that it would involve 100 workers and two cranes, each 45 metres (150ft) tall, on platforms.

Mario Lolli Ghetti, head of culture for the Tuscan region, said that disruption would be kept to a minimum. Antonio Natali, director of the Uffizi, said that the reconstruction was vital.

Annamaria Petrioli Tofani, the former director, said that Florentines should “mobilise themselves and wake up to how a major monument is being altered”.

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