A “big deal” tax lawyer and former Harvard professor with decades of experience has delved deep into the allegations of income tax avoidance at Vanguard and penciled out an estimate of $34.6 billion owed in back taxes since 2007.

“If the IRS were to pursue this matter, it will prevail in court,” University of Michigan Law School Prof. Reuven S. Avi-Yonah wrote in a report cited by a recent Philly.com article.

Vanguard continues to “believe the case is without merit and decline to comment further,” spokesman John Woerth said in the article, and experts speculate that not only will Vanguard have compelling arguments against the allegations but that any potential penalty would be settled for far less than the billions being proposed.

Nevertheless, there’s a growing number of experts who think David Danon may have a legitimate case.

“‘It seems like a pretty clear-cut case,” New York business tax attorney Robert Willens said in the article. “‘I know Vanguard says they have meritorious defenses and special circumstances that might excuse them. But the principle in law is that they have to deal with their funds at arm’s length,’ and the funds must pay Vanguard’s management company fees as if they were paying an independent contractor, not a special low price.”

The essence of the complaint attacks the very core of Vanguard’s wildly popular mutual funds: their extremely low fees.

The courts have yet to rule on whether to dismiss David Danon’s complaints against his former employer, and the IRS and Securities and Exchange Commission have also been afforded opportunities to investigate the matter.

Read more details about Avi-Yonah’s report and other expert opinions on Philly.com here.