The group, which includes Coventry-based Evtec, Solihull-based Artifex and the Confederation of British Metal Forming, said suppliers faced rising energy costs, growing overseas competition and increasing pressure from vehicle manufacturers sourcing components abroad.
David Roberts, chairman of Evtec, said manufacturers believed high-volume car production in the UK was entering a decade of decline.
Speaking on Politics Midlands, he said: “We want to take action today rather than wait for that to happen.”
Roberts argued there was a “very big opportunity” for manufacturers to redirect skills, factories and investment into the UK’s expanding aerospace and defence sectors.
The letter said automotive suppliers already possessed many of the capabilities needed to support aircraft, defence equipment and military vehicles, but warned firms needed help to make the transition.
“The capacity exists. The people exist. What is missing is the bridge,” it said.