(Bloomberg) -- Rivian Automotive Inc. fell after Amazon.com Inc., one of its biggest backers and customers, agreed to buy battery-electric delivery vans from rival automaker Stellantis NV.

The first vehicles under the new order are due next year, Stellantis said Wednesday. It didn’t disclose the size of the deal, but Chief Executive Officer Carlos Tavares said the order was a “significant number.”

Read more: Amazon, Stellantis team up on software, delivery vehicles 

The development is a threat to Rivian, which has been seen as a front-runner in a large pack of EV startups chasing market incumbent Tesla Inc., in part because of the backing of Amazon. The tech giant in 2019 ordered 100,000 electric delivery vans from Rivian, the first 10,000 of which are due by the end of this year.

Shares of Irvine, California-based Rivian tumbled as much as 7.2%, the biggest intraday drop in more than two weeks. Stellantis’s U.S. shares rose by as much as 3.2%. U.S. electric vehicle companies have fallen for a second day amid signs of deepening competition in the sector.

The decline adds to Rivian’s recent slide after the stock briefly topped $100 billion in market value -- at the time more valuable than Ford Motor Co., also an investor, and General Motors Co. -- following its November listing.

Rivian did not respond to a request for comment.

According to Rivian’s Oct. 1 regulatory filing, the e-commerce giant has exclusive rights to Rivian’s vans for four years, from the point of delivery of the first unit. Deliveries commenced in December. Amazon has right of first refusal to any vans built for an additional two years after that. 

Read more: Rivian details $1 billion loss, Amazon deal in IPO filing

However, the terms of the deal outlined in the filing allow Amazon to work with other automakers on electric delivery vans.

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