Mumbai: Bharti Airtel Ltd, India's top mobile services firm, said its cellular user base crossed 40 million, making it the 10th company in the world to achieve the milestone in a single country.

The Sunil Mittal-controlled Bharti Airtel had 38.9 million mobile subscribers at the end of April, according to data provided by Cellular Operators' Association of India (COAI).

One of the earliest entrants to the sector after India opened the telecom sector to private players, Bharti took 11 years to reach 20 million subscribers in 2006 but just 13 months more to double the figure, the company said.

Bharti, which provides mobile services in all the 23 zones or circles that make up India's telecom market, said its share of the wireless user base increased to 23.2 per cent from 20.4 per cent end-March.

Bharti, in which Singapore's SingTel owns a 30.8 per cent stake, competes with 11 other players in India's zooming market.

Reliance Communications is India's second-largest mobile services firm, followed by unlisted state-owned Bharat Sanchar Nigam Ltd.

Britain's Vodafone Plc earlier this year bought a controlling stake in the country's No 4 mobile firm Hutchison Essar.

India is the world's fastest-growing mobile market, adding more than six million new users every month lured by low call rates. The country had 166.05 million wireless subscribers end-March.

Analysts and industry experts expect the mobile user base to triple in the next five years as only 15 per cent of India's 1.1 billion population own a mobile phone, compared with 36 per cent in China.