Ratan Tata
Cyrus Mistry, in his letter, had blamed Ratan Tata for all the wrong foreign strategies taken up by the firm. Pictured: Ratan TataReuters

Ratan Tata, chairman emeritus of Tata Sons, has bought a stake in online cab aggregator Ola.

However, Ola did not reveal the amount invested by Tata even though it has confirmed the investment, Reuters reported.

Tata's investments in start-ups are not big, they are considered as valuable. 

Of late, Ratan Tata has shown a strong interest in start-ups, investing in companies such as online retailer Snapdeal and Chinese handset maker Xiaomi.

Paytm, Urban Ladder, Bluestone and CarDekho are the other start-ups which have attracted the interest of the Tata Group's former chief.

Ola is now the third-most valuable, venture-backed company in India after Flipkart and Snapdeal. After raising ₹2,500 crore in April, the Bengaluru-based company saw its valuation rise to ₹15,600 crore, The Economic Times reported.

In the previous round of funding, Japan's SoftBank invested $210 million in the company in October 2014. 

Founded in 2011, Ola has expanded its operations to more than 65 cities and acquired its rival TaxiForSure in March. Ola claims to have 80% of market share after merging TaxiForSure.

Ola, which currently faces competition with US-based online cab aggregator Uber, plans to expand to about 200 smaller towns and cities in India.

Uber is currently the second most valuable start-up in the world at $41 billion after Chinese smartphone maker Xiaomi Inc. India is an important market for Uber, as it has only a minor presence in China.