Global rating agency Standard & Poor's has kept its rating for India unchanged at the lowest investment grade "BBB-minus" with a "stable" outlook, citing low income and weak government's fiscal position.

However, the rating agency does not expect a revision in the rating in 2015 or next year, taking into account its "current set of forecasts".

S&P's move will "disappoint" Prime Minister Narendra Modi's government, as it was looking for higher rating for keeping fiscal deficit under control and improvement in the economic situation, Reuters reported.

In April, another rating firm -- Moody's Investors Service -- also maintained it's lowest investment grade rating for India. However, it revised its outlook from "stable" to "positive", saying the government's steps will result in improved economic growth.

Similar to Moody's, S&P also hailed the steps taken by the Modi government to improve the business environment, labour conditions and the energy sector. However, it cited increasing revenue and a check on subsidy expenditure as the major fiscal challenges for the government.

"Although we expect the new administration to pursue its stated fiscal consolidation programme, we foresee that planned revenues may not fully materialise and subsidy cuts may be delayed," S&P said in a statement on Monday.

"Overall, we believe public finances are set to remain key rating constraints for some time," it said.

Fitch Ratings has also kept its rating for India at "BBB-minus" with a "stable" outlook.

S&P projects the Indian economy to grow by 7.4% this year. It expects the average growth of the economy to remain "just under" 8% from 2015 to 2018.