Aston University's Prof David Bailey said he hoped firms would still support UK steel but warned Tata would be less attractive to buyers if it loses sales.
He added something had to be done soon to end the uncertainty.
Nissan is said to get 45% of its sheet steel from Port Talbot while Tata across south Wales supplies Vauxhall.
The two industries could not be experiencing more different fortunes at the moment.
While the car industry is booming, steel is at crisis point.
"I certainly hope that manufacturers and component suppliers in particular will stick with Tata in the short-term in the hope that this can be sorted out and we can save the British steel industry", Prof Bailey told BBC Wales.
However he said he was "pretty sure component manufacturers will be looking at other options" particularly sourcing replacement high quality steel from Europe.
Aberavon MP Stephen Kinnock urged Business Secretary Sajid Javid, to hold talks with Tata's customers to convince them to keep buying from the company.
Mr Javid responded in a House of Commons debate that he was holding discussions with firms but it would be wrong to go into details.
Uncertainty over Tata's future means parts of the car industry will look at options abroad for high quality steel, a leading economist has warned.