Sam Altman, chief executive of OpenAI, has quietly built up a portfolio of personal investments valued at almost $3 billion in technology companies, some of which do business with his artificial intelligence firm.

Altman, 39, has become one of Silicon Valley’s most prolific investors with holdings in more than 400 companies, including Airbnb, Stripe and Reddit, managed by his family office. The scale of his investment empire was first reported by The Wall Street Journal.

Altman has no stake in OpenAI, saying he does not want the seductions of wealth to corrupt the development of AI, and is paid an annual salary of $65,000. However, he is building a fortune via his personal investment portfolio, which has been funded by a debt line from JP Morgan. It includes holdings in companies which do business with OpenAI, raising questions about potential conflicts of interest.

In one example, OpenAI announced a partnership with Reddit, the social media platform, which sent the company’s shares up by more than 10 per cent, increasing the value of Altman’s personal Reddit stake.

A blog post about the partnership, published in May, included a disclosure that Altman is a shareholder in Reddit. OpenAI, which developed the AI system ChatGPT, said the partnership was led by Brad Lightcap, its chief operating officer, and approved by its independent board of directors.

Other companies in which Altman has made personal investments include Exowatt, a start-up that aims to modernise data centres for the AI era, and Helion, a company planning to build nuclear-fusion power plants. OpenAI is in talks with Helion to buy large quantities of electricity to power data centres, and Altman has recused himself from negotiations, the WSJ reported.

Altman is a Stanford University dropout who arrived on the tech scene as the founder of Loopt, a mobile social networking service. He was president of Y Combinator, a San Francisco-based start-up accelerator, for five years, before becoming chief executive of OpenAI in 2019.

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Other companies he has invested in include Retro, which aims to extend healthy human lifespans by ten years. He has also co-founded a venture firm, Apollo Projects, with his brother Max.

Last month Altman and his husband, Oliver Mulherin, a software engineer, signed a pledge to give away most of their wealth to philanthropic clauses. “We intend to focus our giving on supporting technology that helps create abundance for people, so that they can then build the scaffolding even higher,” they wrote in an open letter.

Bret Taylor, chairman of OpenAI’s board, told the WSJ that Altman has “consistently followed policies and been transparent about his investments”.

He said: “Sam is fully focused on his role as chief executive. We carefully manage any potential conflicts and always put OpenAI and our mission first. Our fully independent audit committee reviews all potential conflicts involving directors and officers to ensure the best outcomes for OpenAI.”

Altman was briefly sacked as chief executive of OpenAI in November, with the board declaring it no longer had confidence in his leadership. The board claimed that Altman “was not consistently candid in his communications”. He was reinstated five days later, following a backlash from OpenAI employees.

News Corporation, ultimate owner of The Times, last month became the latest publisher to strike a deal allowing OpenAI to use news content from its publications.