The final HSBC India Services purchasing managers' index, compiled by S&P Global, fell to 60.2 last month from April's 60.8, confounding a preliminary reading for a rise to 61.4.
However, it remained above the 50 mark separating growth from contraction for the 34th straight month.
"India's service activity rose at a slightly softer pace in May, with domestic new orders easing slightly, but remaining robust, implying strong demand conditions and successful advertising," noted Maitreyi Das, global economist at HSBC.
