Cameron, accompanied by a large trade team, is on a three-day visit.
"India's rise is going to be one of the great phenomena of this century and it is incredibly impressive to see," Cameron commented during a meeting at the Unilever headquarters in Mumbai, the BBC reported.
"Britain wants to be your partner of choice. We've only just started on the sort of partnership that we could build. As far as I'm concerned, the sky is the limit."
Cameron's office said the trade delegation accompanying him is the largest a prime minister has taken on a foreign trip.
The two countries want to more than double their bilateral merchandise trade to $34 billion by 2015.
The British leader spoke with Indian industry leaders in Mumbai.
Cameron urged India to further open up its insurance, banking and retail sectors and simplify procedures for foreign investors, The Wall Street Journal reported.
This is the second India visit for Cameron since becoming prime minister in 2010.
Cameron's visit comes at a time when United Kingdom visa rules have been tightened, but he gave assurances there would be no limit on admission to British universities for Indian students proficient in English.