"Congratulations @narendramodi on victory in India's elections. Keen to work together to get the most from UK-India relationship," tweeted Cameron, who was among the first world leaders to greet Modi.
While an official Downing Street statement will be issued later, UK Foreign Office issued its statement on the close relationship between the two countries.
UK foreign secretary William Hauge said: "I congratulate Narendra Modi and the BJP on their success and look forward to forging an even closer partnership with India in the months ahead."
"With 815 million eligible voters, 915,000 polling stations, and nine days of voting over five weeks, India has seen the largest parliamentary election in the world.
"The UK has strong ties with India and the British Government looks forward to working with the new Indian Government to build on this relationship and deliver security, growth and prosperity for both our nations," he said.
Modi is all set to become the next Prime Minister of India as his party is heading towards a majority on its own and form the next government.