"This is likely to be the first instance within Asia since the implementation of the Basel III framework. It is also important at this time because a number of Indian banks - mostly state-owned, are considering raising fresh regulatory capital in the international market, in light of the capital pressures on the sector," it said in a release.
Fitch said the authorities face a dilemma and their response could set a credit precedent.
It further said that the compounding losses could exacerbate the risk for investors.
Government has already injected Rs 700 crore into UBI this financial year.
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Kolkata-based UBI's credit profile has weakened sharply, and has been worse than at other state-owned banks.
Accelerating asset-quality problems raised its gross NPA ratio to 10.8 per cent in December, up sharply from 5.6 per cent in June, 2013.
The net loss for the first nine months of this fiscal stood at Rs 1,683 crore as against a net profit of Rs 361 crore in the similar year-ago period.
The gross NPAs (non-performing assets) at the end of the third quarter was 10.82 per cent and net NPA, 7.44 per cent.