Williams was featured in a video announcement on Tuesday, in which Lewis was accused of leveraging insider information to "shower gifts on his friends and lovers", per BBC News.
The attorney also said that the Spurs owner "has been indicted and will face justice".
He added: "We allege that, for years, Joe Lewis abused his access to corporate board rooms and repeatedly provided inside information to his romantic partners, his personal assistants, his private pilots, and his friends."
It is alleged the information passed onto Lewis' acquaintances earned them millions on the US stock market.
Williams said: "Thanks to Lewis, those bets were a sure thing."
A member of Lewis' legal team hit out at an "egregious error of judgement" in bringing charges against the billionaire.
The Spurs owner is one of Britain's richest men and has owned the club since 2002, when he paid £22m ($28m) to previous owner Alan Sugar.
It has been reported by New York Times journalist Tariq Panja that Lewis being convicted of the allegations may force him to sell the club, as Premier League rules state that a person with a criminal conviction cannot hold a stake in a top-flight side.
A Tottenham spokesperson said in a statement: "This is a legal matter unconnected with the club and as such we have no comment."