WASHINGTON — Looking to hide a few million from the taxman? Switzerland might not be your best choice anymore.

The U.S. and Switzerland detailed an agreement yesterday for the IRS to receive information on thousands of accounts in banking giant UBS AG — accounts suspected of holding Americans' undeclared assets.

The agreement breaks through the famed Swiss tradition of banking secrecy and is expected to prod thousands more UBS clients in America to voluntarily disclose their financial details to the Internal Revenue Service, lest they be pursued later.

"This is no mere keyhole into the hidden world of bank secrecy," said IRS Commissioner Doug Shulman. "This agreement represents a major step forward with the IRS' efforts to pierce the veil of bank secrecy and combat offshore tax evasion."

Shulman said the accounts held $18 billion at one time, though many have since been closed.

The Swiss, known worldwide for keeping bank accounts secret, said UBS had no real choice in turning over the names.

Justice Minister Eveline Widmer-Schlumpf told a news conference in Bern that the deal lifts the threat of criminal prosecution, which not only would have endangered the bank's existence but would have dealt a severe blow to the Alpine nation's economy.

"There was no alternative to this solution," she said.

The agreement is part of the Obama administration's stepped-up efforts to go after wealthy tax dodgers hiding assets in offshore accounts, an initiative that promises to yield many more prosecutions, Shulman said.

UBS has an estimated 52,000 accounts held by U.S. customers. The IRS chief said the 4,450 accounts being identified were the ones most suspected of containing undeclared assets. Many of the rest are held by people who have complied with the law and paid their taxes, he said.

Tax experts said the agreement should terrify Americans who had been able to hide assets in offshore accounts for generations with little fear of being caught.

"This is critically important because the Swiss caved," said Tom Cardamone, managing director of Global Financial Integrity, a Washington-based group that advocates tougher policies against international money laundering. "They agreed to give up names, so in that context, this is a real body blow to Swiss banking secrecy."

UBS shares closed 0.9 percent lower at 16.74 Swiss francs ($15.74) on the Zurich exchange.