The lower growth estimate of 5.5 per cent is down by about one percentage point from the earlier one, it said in a note.
"The lack of monsoon rains has affected India, for which agriculture still forms a substantial part of the economy. Additionally, the more cautious investor sentiment globally has seen potential investors become more critical of India's policy and infrastructure shortcomings," it said.
The lowering of the growth estimate comes even as the government under new Finance Minister P Chidambaram has taken a slew of reform measures and stated its commitment to do more on the reforms front.
The measures, which have run into political controversy, included hike in diesel prices, putting a cap on the number of subsidised cooking gas cylinders a family can use, liberalising foreign holding norms in aviation, multi-brand retail, non-news broadcasting and power exchanges sectors, and divesting its stake in five companies.
"At this stage it is still uncertain whether these measures can be implemented," S&P, which had downgraded its outlook on the country's sovereign rating to negative and then threatened to junk it in June failing concrete measures from the Government, had said in reaction to the reform measures.
In the revised estimates published today, the agency has also forecast lower growths for many other countries, including China.