In that trial, Berlusconi, 70, is accused of using offshore companies to buy film rights between 1994 and 1999 and then reselling them at inflated prices to Mediaset, which as a result paid less tax in Italy.
Mills, the estranged husband of Tessa Jowell, Britain's culture secretary, was Berlusconi's tax lawyer at the time of the alleged business transactions.
The prosecution has only until November to take the case, which opened in November 2006, through Italy's three-tiered court system.
"The problem is that the court will only sit on two days a week," Alan Fisher, Al Jazeera's correspondent is Milan, said.
"The defence knowing that the time is running out, that there is this deadline, will call 83 witnesses and has lodged more than 1,000 pages of documents so the court will simply not get done in time and they will walk away free men," he added.
Mills, 62, who specialises in offshore tax havens, initially admitted receiving money from Berlusconi "in recognition" for his work.
He later recanted, saying the money was paid to him by an Italian shipbuilder, Diego Attanasio.
Berlusconi has consistently denied wrongdoing. A fierce anti-Communist, he has long alleged that "red judges" in the Milan judiciary are biased against him.
Berlusconi also faces trial in Spain, where he is accused of covering up tax fraud of $142m during the early 1990s in connection with his holdings in Telecinco, a Spanish television channel.
His Mediaset company, which owns three national television channels and an advertising group in Italy, controls Telecinco.
Berlusconi is Italy's richest man, with a fortune estimated at $11bn. His vast business empire also includes a publishing conglomerate, insurance companies and department stores.
Cases related to Berlusconi's business dealings have landed him in court for eight separate trials on charges including corruption, tax fraud, false accounting and illegally financing political parties.
He has never been definitively convicted.