Travis Kalanick has stepped down as chief executive of Uber, the ride-hailing service that he helped found in 2009 and that he built into a transportation colossus, after a shareholder revolt made it untenable for him to stay on at the company.

Kalanick's exit came under pressure after hours of drama involving Uber's investors, according to two people with knowledge of the situation, who asked to remain anonymous because the details were confidential.

Earlier Tuesday, five of Uber's major investors demanded that the chief executive resign immediately. The investors included one of Uber's biggest shareholders, the venture capital firm Benchmark, which has one of its partners, Bill Gurley, on Uber's board.

The investors made their demand for Kalanick to step down in a letter delivered to the chief executive while he was in Chicago, said the people with knowledge of the situation.

In the letter, titled "Moving Uber Forward" and obtained by The New York Times, the investors wrote to Kalanick that he must immediately leave and that the company needed a change in leadership.

Kalanick, 40, consulted with at least one Uber board member, and after long discussions with some of the investors, he agreed to step down. He will remain on Uber's board of directors.

"I love Uber more than anything in the world and at this difficult moment in my personal life I have accepted the investors' request to step aside so that Uber can go back to building rather than be distracted with another fight," Kalanick said in a statement.

Uber's board said in a statement that Kalanick had "always put Uber first" and that his stepping down as chief executive would give the company "room to fully embrace this new chapter in Uber's history." An Uber spokesman declined to comment further.

The move caps months of questions over the leadership of Uber, which has become a prime example of Silicon Valley startup culture gone awry. The company has been exposed this year as having a workplace culture that is rife with sexual harassment and discrimination, and has pushed the envelope in dealing with law enforcement and even partners.

That tone was set by Kalanick, who has aggressively turned the company into the world's dominant ride-hailing service and up-ended the transportation industry around the globe.

Kalanick's troubles began earlier this year after a former Uber engineer detailed what she said was sexual harassment at the company, opening the floodgates for more complaints and spurring internal investigations.

In addition, Uber has been dealing with an intellectual property lawsuit from Waymo, the self-driving car business that operates under Google's parent company, and a federal inquiry into a software tool that Uber used to sidestep some law enforcement.

Ousting executives

Uber has been trying to move past its difficult history, which has grown inextricably tied to Kalanick. In recent months, Uber has fired more than 20 employees after an investigation into the company's culture, embarked on major changes to professionalise its workplace, and is searching for new executives including a chief operating officer.

Kalanick last week said he would take an indefinite leave of absence from Uber, partly to work on himself and to grieve for his mother, who died last month in a boating accident. He said Uber's day-to-day management would fall to a committee of more than 10 executives.

But the shareholder letter indicated that his taking time off was not enough for some investors who have pumped millions of dollars into the ride-hailing company, which has seen its valuation swell to nearly $US70 billion ($93 billion). For them, Kalanick had to go.

The five shareholders who demanded Kalanick's resignation include some of the technology industry's most prestigious venture capital firms, which invested in Uber at an early stage of the company's life, as well as a mutual fund firm.

Apart from Benchmark, they are First Round Capital, Lowercase Capital, Menlo Ventures and Fidelity Investments, which together own more than a quarter of Uber's stock. Because some of the investors hold a type of stock that endows them with an outsize number of votes, they have about 40 percent of Uber's voting power.

Leadership vacuum

Kalanick's resignation opens questions of who may take over Uber, especially since the company has been so moulded in his image. And Kalanick will probably remain a presence there since he still retains control of a majority of Uber's voting shares.

Taking a startup chief executive to task so publicly is relatively unusual in Silicon Valley, where investors often praise entrepreneurs and their aggressiveness, especially if their companies are growing fast. It is only when those startups are in a precarious position or are declining that shareholders move to protect their investment.

Uber has been valued at $US69 billion, making it the world's most valuable private tech company. The company has raised more than $US11 billion from investors since its founding in 2009.

Uber's investors also include TPG Capital, the Public Investment Fund of Saudi Arabia, mutual fund giants like BlackRock and wealthy clients of firms like Morgan Stanley and Goldman Sachs.

Despite recent turmoil, Uber's business is growing. Revenue increased to $US3.4 billion in the first quarter, while losses narrowed - though they remain substantial at $US708 million.

New York Times, Bloomberg