Ratan Tata, the former Tata Group chairman who transformed a staid and sprawling Indian conglomerate into a global powerhouse with a string of high-profile acquisitions, has died at the age of 86.

Mr Tata, who ran the conglomerate for more than 20 years as chairman, had been undergoing intensive care in a Mumbai hospital, two sources with direct knowledge of his medical situation told Reuters earlier on Wednesday.

"It is with a profound sense of loss that we bid farewell to Mr Ratan Naval Tata, a truly uncommon leader whose immeasurable contributions have shaped not only the Tata Group but also the very fabric of our nation," the Tata Group said in a statement.

Mr Tata "was a visionary business leader, a compassionate soul and an extraordinary human being," Indian Prime Minister Narendra Modi said on social media platform X.

"Extremely pained by his passing away. My thoughts are with his family, friends and admirers in this sad hour."

In his statement, Mr Modi said that Mr Tata's legacy stretches far beyond the business world.

After graduating with a degree in architecture at Cornell University, he returned to India and in 1962 began working for the group his great-grandfather had founded nearly a century earlier.

He started out in 1962 at TISCO, now known as Tata, staying in a hostel for apprentices and working on the shop floor near blast furnaces.

He worked in several Tata companies and made his mark by erasing losses and increasing market share at group unit National Radio & Electronics Company.

He took over the family empire in 1991, riding the wave of the radical free-market reforms India had just unleashed that year.

In one of his first steps, Mr Tata sought to rein in the power of some heads of Tata Group's companies, enforcing retirement ages, promoting younger people to senior positions and ramping up control over companies.

In 2004, he took the conglomerate's software outsourcing arm Tata Consultancy Services public at a time when many were nervous about investing in technology stocks.

That same year, he declared he wanted Tata Group to "spread its wings far beyond India" and become "at home in the world" — prompting a world tour to buy up major brands, including Britain's Tetley Tea and Anglo-Dutch steel firm Corus.

It "was the quest for growth and changing the ground rules to say that we could grow by acquisitions which earlier we had never done," he said in an interview with the Stanford Graduate School of Business in 2013.

His pet projects at Tata Motors included the Indica — the first car model designed and built in India — as well as the Nano, touted as the world's cheapest car. He contributed initial sketches for both models.

The Indica was a commercial success. 

However the Nano, billed as the world's cheapest car, stuttered to a halt after finding few takers in status-conscious India, where consumers flinched at owning a so-called "poor man's car".

But the losses paled before his triumphs, which saw the Tata Group's revenues rise from about $US6 billion ($9 billion) to $US100 billion, as the company's reach grew to more than 100 countries.

A licensed pilot who would occasionally fly the company plane, Mr Tata never married and was known for his quiet demeanour, relatively modest lifestyle and philanthropic work.

About two-thirds of share capital of Tata Sons, the group's holding company, is held by philanthropic trusts.

His leadership at Tata was not without controversy — most notably a bitter public feud after the company ousted Cyrus Mistry, a scion of the billionaire Shapoorji Pallonji clan, as chairman of Tata Sons in 2016.

The Tata Group said Mr Mistry had failed to turnaround poorly performing businesses while he accused Mr Tata, who was chairman emeritus of the conglomerate, of interfering and creating an alternate power centre at the group.

After he stepped back from the Tata Group, Mr Tata became known as a prominent investor in Indian startups, backing a plethora of companies including digital payments firm Paytm PAYT.NS, Ola Electric, a unit of ride hailing firm Ola, and home and beauty services provider Urban Company.

Among his many awards, he received the Padma Vibhushan, India's second highest civilian honour, in 2008 for exceptional and distinguished service in trade and industry.

Reuters/AFP