By Greg Morcroft

NEW YORK (Dow Jones) - Huntington Bancshares stock fell 13% on Monday after Citigroup analysts downgraded the Ohio-based regional bank and said its research shows its capital levels are potentially more troubling than most rivals.

Citi analysts downgraded shares of Regions Financial (RF), Zions Bancorp (ZION) and Marshall & Illsley (MI@) to hold for the same reasons.

Those three shares all fell, but not as sharply as Huntington did.

Citi analysts said the preformed their own "stress test" on a number of regional banks, and used criteria they believe that the government will use in it own well publicized tests of banks with at least $100 billion in assets.

"Given our belief that the stress-test announced by the US Treasury that will be performed on banks exceeding $100 bln will ultimately be applied more widespread, we performed a stress-test on our regional bank coverage to help formulate a view of how the banks are positioned for this from a capital standpoint," the analysts said in an extensive 78-page report delivered to clients on Monday.

The research determined that Synovus (SNV), TCF Financial and New York Community Bancorp (NYB) are best positioned among the regional banks to perform well on a stress test.

Those shares all rose Monday.

To run their test, Citi analysts said the evaluated several data sets, including their regulatory issues, geographic locations, loan-to-value-ratios on the mortgage loans the banks have made and cumulative defaults to date, to estimate a worst-case scenario for total losses during the current downturn.

The analysts said, "it is important to note that the stress test we performed is severe in that it assumes all future losses are recognized in 2009, which we believe is a lower probability scenario, and would require substantial deterioration from current economic conditions which are already severely stressed."