The agency has also downgraded the bank's subordinated medium-term notes (MTN) programme and the junior subordinated MTN programme to (P)B1 and (P)B2 from (P)Ba3 and (P)B1, respectively.
"We consider that IDBI Bank's standalone credit profile continues to be negatively impacted by its asset quality issues. Its impaired loan ratio rose to 14.7 per cent at end-March 2015 as against 12.16 per cent a year earlier," it said.
While the pace of new impaired loan formation will slow during the year ending FY16 compared to the levels seen over the last three years, the rating agency expects the rate to remain high.
The agency said a significant amount of corporate loans have already been recognised as impaired loans by the banking system.
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"This situation is especially true for a few large and highly leveraged corporate groups. Such exposures are the key sources of incremental asset quality risk for domestic banks, including IDBI," it said.
IDBI's buffers against further asset quality stress remain weak. Its core tier 1 of 7.29 per cent at end March 2015 is low on an absolute level.
The bank did not receive any capital infusion from the government (Baa3, Positive) this year as it did not meet the government's profitability threshold. The bank's ability to raise external core equity capital remains bleak as well and so capital remains a key credit weakness for it.
The agency said an upward change in the baseline credit assessment is possible if the bank substantially improves its profitability and tier 1 capital position on an internally generated basis or by accessing the capital markets rather than being dependent on equity infusion from the government.
The agency has confirmed IDBI Bank's deposit rating of Baa3, which benefits from four notches of uplift from its BCA of b1, owing to its assumption of a very high probability of government support, in spite of its weak standalone credit profile.