Tesla Inc is postponing its planned robotaxi unveiling to October to allow teams working on the project more time to build additional prototypes, according to people familiar with the decision.

The roughly two-month delay has been communicated internally, said the people, who asked not to be identified because the information hadn’t been publicly announced. The design team was told this week to rework certain elements of the car, one of the people said.

Chief executive officer Elon Musk set the initial Aug 8 date for the event months ago, and optimism about the spectacle has contributed to an 11-day streak of gains that added more than US$257bil to Tesla’s market capitalisation. The stock closed 8.4% lower on Thursday, its largest drop since January.

Musk didn’t respond to requests for comment.

Shares of would-be taxi rivals Uber Technologies Inc and Lyft Inc both surged on the news. Uber’s stock rose 6.1% on Thursday. Lyft shares jumped 4.6%.

The idea of creating an autonomous taxi service has been kicking around Tesla for years, dating at least as far back as when Musk authored a second "master plan” for the company in 2016. The CEO has prioritised the project in recent months over work on an electric vehicle cheaper than Tesla’s most affordable car, the Model 3 sedan.

Musk has talked up Tesla’s work on autonomous-vehicle technology for over a decade and convinced customers to pay thousands of dollars for a suite of features the company markets as Full Self-Driving, or FSD. The name is a misnomer - FSD requires constant supervision and doesn’t render Teslas autonomous - but Musk and top engineers have been increasingly bullish about FSD in recent months as the company’s vehicle sales have slowed.

Tesla delivered 6.6% fewer cars in the first half of the year, despite the company adding a new model - the Cybertruck - to its lineup. The automaker also produced 14% fewer vehicles in the second quarter than it did a year earlier to help curtail swelling inventory.

 - Bloomberg