TOKYO (Reuters) -SoftBank's plans to set up a joint venture with OpenAI to bring artificial intelligence services to corporate customers in Japan are significantly behind schedule, according to a source familiar with the matter.

The joint venture was supposed to be set up this summer but preparations are taking longer than anticipated, with an update on progress expected in November, the source said, declining to be named as the details are not public.

SoftBank said preparations were proceeding but declined to comment on details. OpenAI did not immediately respond to a request for comment.

SoftBank CEO Masayoshi Son announced the venture, SB OpenAI Japan, with OpenAI CEO Sam Altman in February. SoftBank said the venture would be owned by OpenAI and a company established by SoftBank and its domestic telecoms unit.

At a shareholder meeting in June, the CEO of the telecoms business, Junichi Miyakawa, said July-end was the target to set up the venture. The products the business will offer are in discussion, Miyakawa said.

After a period of retrenchment due to a series of soured tech investments, Son is making splashy investments again, backing OpenAI and other ventures related to AI.

The $500 billion Stargate project to develop data centres in the U.S. has been delayed by protracted negotiations with other parties and on decisions related to locations, SoftBank's chief financial officer Yoshimitsu Goto said last month.