"The rating action reflects our assumption of a lower level of support from the government following its announcement that indicates that it wishes to differentiate between the state-owned banks when distributing capital," the rating agency said in a release.
The rating agency downgraded local and foreign currency deposit ratings of the two banks to Ba1 and Baa3.
Recently, the government infused Rs 6,990 crore into nine out 28 state-run banks, based on their return on equity and return on assets against a budgeted Rs 11,200 crore.
The agency also downgraded IOB's senior unsecured debt, issued from its Hong Kong branch, to Ba1 from Baa3.
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The report, however, said it continues to assume a very high probability that the government would support these two banks.
It further said the changes in the government's policy indicates that standalone quality of state-run banks has become a more important consideration for senior unsecured and deposit ratings of the banks.
It said both these lenders have the weakest standalone credit profiles among the banks it rates here as indicated by their baseline credit assessments at b3 for Central Bank and b2 for IOB.
"Consequently, to reflect the impact of the government's establishment of more selective criteria, we have lowered the notches of support provided to these two banks," the rating agency said.