In a statement posted on the David Chipperfield Architects website, the firm claimed it had "played no part in the artistic supervision" of the building's fit-out, and accused Milan City Council of ignoring problems that could have been rectified.
"The Milan City Council is opening the doors of the museum despite David Chipperfield Architects having played no part in the artistic supervision of the design of the permanent exhibition and despite the fact that the Milan City Council has not attempted to rectify the defects in the realisation of the Museum that David Chipperfield Architects has been calling attention to for the last two years," it said.
These alleged defects include a stone floor that Chipperfield described as "unacceptable" earlier this year.
At the time, the architect said the stone contract was not properly supervised, and the resulting surface was scratched, stained and misaligned. He offered to cover half of the cost of replacing it out of the fees still owed to him by the client.
But six months later the matter is still unresolved, prompting Chipperfield to decline an invitation from mayor Giuliano Pisapia to attend the opening of Museo delle Culture (MUDEC) on 27 October.
According to Chipperfield, legal proceedings are currently underway to "establish both the existence of and the liability for the defects" within the building.
Like French architect Jean Nouvel – who sought a court mandate to have his Paris concert hall adapted to comply with his original design – Chipperfield believes legal proceedings could force the city to finish the museum to a higher standard.
"Lawyers representing the parties concerned are currently in confidential negotiations through the courts in order to understand whether a settlement can be reached so as to draw this controversy to a close and finish MUDEC in the way the city of Milan deserves," said the firm.
The museum occupies the former Ansaldo factory, located south-west of the city in the creative district surrounding Via Tortona.
Designed as an mix of two-, three- and four-storey volumes framed by courtyards and passages, it accommodates the Centre for Advanced Studies of Visual Art (CASVA), the Centre of Non-European Cultures and the New Archaeological Museum.
The museum had a soft launch in March, timed to coincide with the opening of the 2015 World Expo. It was at this time that first reports surfaced of issues between architect and client.
Chipperfield – whose other completed museum projects include the Neues Museum in Berlin and the Hepworth Wakefield gallery in northern England – described it as "a pathetic dispute at the end of 15 years work" and urged taxpayers to demand the building be finished to the standard that was promised.
Museum photography is by Oskar Da Riz.