Motorola faces the prospect of being forced to remove its handsets from sale in Germany and to recall phones already bought by business customers.
A local court has ruled that the antennas used by the Google-owned business infringe a patent owned by a German laser specialist.
The firm, LPKF, now has the right to block the products from sale unless Google comes to some other agreement.
It told the BBC it had not yet decided how to proceed.
A German patent consultant said it was not a simple decision to make.
"If LPKF wants to enforce the action it faces economic risk," said Florian Mueller, who has previously advised Microsoft and Oracle.
"If it makes Motorola pull and recall products today, LPKF could later be liable for wrongful enforcement damages in a year or so if Motorola appeals and ultimately prevails."
Google is in the process of selling its Motorola business to the Chinese tech firm Lenovo, but the deal has yet to be completed.
"We are disappointed in the decision but Motorola has taken steps to avoid any interruption in supply," said a spokesman for the firm.
LPKF has signalled that it could also take action against other tech firms.
The German company helped pioneer a process called Laser Direct Structuring (LDS), which offers a way to create antenna patterns.
It involves using a laser beam to create microscopic pits and undercuts on plastic, to which a metal coating can then be anchored.
This can prove cheaper than other options, such as hot stamping the pattern or using injection moulding, especially when more complex 3D antenna designs are desired to help keep components small and hidden inside a device.
The German firm's LDS patent was ruled invalid last year in China, which has encouraged others to use the process without paying LPKF a licence fee. The company is in the process of appealing against the decision.
"The more attractive a patent is, the harder you have to work to defend it," said Dr Ingo Bretthauer, LPKF's chief executive.
"We will continue to fight for our patent in China and systematically take action against infringers outside China. This is part and parcel of a technology company's everyday business."