The Long-Term Issuer Rating has been downgraded to 'IND AA-' with a stable outlook on "deterioration in its asset quality as well as continued weakness in its funding profile", it said in a statement.
It added that following the lifting of sanctions on India-Iran bilateral oil trade, UCO Bank (which was used to facilitate the trade) has seen a sharp 23 per cent decline in its current account deposits.
Even on a standalone basis, UCO's profitability and capital buffers appear weaker than those of its peers.
It also downgraded United Bank of India's Additional Tier-I bonds to 'IND BBB' with a negative outlook, but affirmed the long term rating on the Kolkata-headquartered bank at 'AA-' with a stable outlook.
The bonds have been downgraded on heightened risk of coupon deferral, led by continued pressure on profitability and a negative revenue reserve, it said.
It also affirmed Vijaya Bank's rating at 'IND AA-' placing its outlook at stable and Dena Bank at 'IND AA-' with a stable outlook.