Jan 30 (Reuters) - SoftBank is in talks to lead a funding round of up to $40 billion in OpenAI at a post-money valuation of $300 billion, sources told Reuters, in what could be the largest ever single funding round for a private company.

SoftBank has offered a term sheet that would value OpenAI at a pre-money valuation, meaning the valuation without the funding of the current round, at $260 billion. This marks a jump for the ChatGPT maker from a few months ago, when it was last valued at $150 billion, added the sources, who requested anonymity to discuss private matters.

OpenAI and SoftBank declined to comment. The Wall Street Journal reported earlier on Thursday that OpenAI was in talks with SoftBank for an investment round to raise nearly $40 billion that would value the AI startup at up to $340 billion.

SoftBank 9984.T could invest $15 billion to $25 billion directly into Microsoft-backed MSFT.O OpenAI, some of which may be used to pay for OpenAI's commitment to Stargate, one of the sources added.

Stargate, a joint venture by Oracle ORCL.N, OpenAI and SoftBank, plans to invest up to $500 billion to help the U.S. stay ahead of China and other rivals in the global AI race.

SoftBank's investment would be on top of the $15 billion it has already committed to Stargate, the person said, adding the talks are at an early stage.