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(CNN Business) —  

Forget about drones and armies of people driving Amazon trucks. One big Wall Street firm thinks Amazon’s plan to build up its air freight delivery service may be what really winds up hurting UPS and FedEx.

Morgan Stanley analyst Ravi Shanker said in a report Tuesday that both delivery giants could lose 10% of their revenue to Amazon Air by 2025. Shanker cut his price targets on UPS (UPS) and FedEx (FDX) and both stocks plunged more than 6% as a result.

It was a particularly gruesome day for transportation stocks due to worries about the broader economy and confusion about the status of US-China trade talks.

The Dow Jones Transportation Average, which includes FedEx and UPS as well as leading airlines, truckers and railroads, fell 4.4% – its worst drop since June 2016.

But Shanker paints a picture that should be extremely worrisome to UPS and FedEx.

He notes that Amazon (AMZN), which currently is leasing 40 cargo jets, could eventually have 100 planes running and estimates that the planned Amazon Air routes could overlap with more than two-thirds of the volume flown by UPS and FedEx.

That’s bad news for both companies since Shanker said that UPS and FedEx each generate nearly 20% of their overall revenue from US air deliveries.

Amazon has made big investments to bulk up Amazon Air. In the past few years, it has bought stakes in two freight delivery airlines – Air Transport Services Group (ATSG) and Atlas Air Worldwide Holdings (AAWW).

It also announced plans to invest $1.5 billion in order to build an Amazon Air hub on more than 900 acres of land that it is leasing near the Cincinnati/Northern Kentucky International Airport. Amazon bought an additional 210 acres earlier this year.

There’s a simple reason why Amazon will want to handle more of its own air deliveries. According to Shanker, Amazon could save between $1 billion and $2 billion next year. That works out to about 3% to 6% of its global shipping costs.

But on a day when the broader market was tanking, investors didn’t seem too enthusiastic about Shanker’s thoughts on Amazon either. Shares of Amazon fell nearly 6%.