Switzerland’s 300 banks have enlisted an army of auditors, lawyers and in-house workers as they race to meet a Dec. 31 deadline on whether to seek U.S. amnesty for helping American clients evade taxes.

Banks in Switzerland, the largest cross-border financial center with $2.2 trillion of assets, are closely examining accounts before joining a disclosure program that’s the broadest assault in a five-year U.S. crackdown on offshore tax evasion.

“The hard work is getting to the right data and cutting through complex systems to get all the facts on the table,” said David Fidan, a partner in Deloitte LLP’s forensic services practice in Switzerland. “That’s very expensive and involves lawyers, forensic accountants and bank employees. It can take 20, 30 or 40 people over four or five months for bigger banks.”

Banks with “reason to believe” they violated tax laws can ask the Justice Department to forgo prosecution. In turn, banks must disclose how they helped Americans hide assets, hand over data on undeclared accounts and pay penalties. Those that don’t apply could face criminal probes like those against 14 banks, including Credit Suisse Group AG, HSBC Holdings Plc and Basler Kantonalbank.

The Swiss government encourages banks to join the program, announced Aug. 29. Yet the Swiss Bankers Association criticizes the program’s cost and vexing questions, such as who qualifies as a U.S. client and what assets are considered untaxed. The answers could determine how much a bank pays in penalties.

“It’s necessary for the banks to do a deep analysis of their clients and the history of those relationships,” said SBA spokeswoman Sindy Schmiegel. “That’s really expensive, and that’s why the program is at the limit of tolerability for the banks. It’s really a painful program.”

At least 33 have announced they will join some form of the program, including 19 cantonal banks, or regional lenders typically owned by regional governments. They include Union Bancaire Privee, the Geneva-based bank founded by Edgar de Picciotto in 1969; Edmond de Rothschild Group, the Geneva-based wealth manager owned by Baron Benjamin de Rothschild; EFG International AG, controlled by Greek billionaire Spiro Latsis and his family; and Bern-based Valiant Holding AG.

Banks expressed confusion over how the Justice Department will calculate penalties and treat lenders that are less culpable in hiding assets from the Internal Revenue Service, said Joshua Milgrim, an attorney at Dechert LLP.

To gain non-prosecution deals, banks must pay 20 percent of the value of accounts not disclosed to the IRS on Aug. 1, 2008, 30 percent for such accounts opened between then and February 2009 and 50 percent for accounts opened afterward.

“The view seems to be that the penalty rates for this program came out way too high,” said Milgrim, who is advising a Swiss bank. “A lot of banks are having difficulty deciding whether to go into a program which doesn’t take into account the level of culpability and instead treats all banks the same.”

Banks are also waiting to find out whether Switzerland’s financial supervisory authority Finma will encourage them to make provisions for costs related to the program and potential fines in this year’s accounts.

Basler Kantonalbank announced a $112 million provision on Dec. 19, while Credit Suisse has set aside 295 million francs for U.S. tax matters.

Finma said it was generally in favor of banks making a provision this year, according to a letter dated Nov. 22 that was published Monday on the website of SonntagsZeitung newspaper. While that letter was non-binding, the regulator may issue further guidance.

“We cannot exclude that we will put out such a recommendation in the future,” Vinzenz Mathys, a spokesman for Bern, Switzerland-based Finma, said Tuesday.

The program has provoked Swiss debate on whether the U.S. went too far in trying to pierce banking secrecy that protected American tax cheats for decades. The crackdown escalated after 2009, when the U.S. charged UBS AG, the biggest Swiss bank, with helping Americans hide $20 billion in assets.

UBS avoided prosecution by admitting it fostered tax evasion and paying $780 million. It handed over 4,700 accounts. The U.S. prosecuted 100 taxpayers, bankers, lawyers and advisers for offshore tax crimes. At least 38,000 taxpayers avoided prosecution by paying back taxes and penalties and disclosing which offshore banks and advisers helped them hide assets.