Before filing a lawsuit against the smartphone maker HTC earlier this week, Apple had warned HTC on several occasions against infringing the iPhone’s patents.

Many other smartphone manufacturers, including Nokia and Motorola, were also contacted by apple as they were working on smartphones which might infringed on patents won by iPhone.

As per a research note by Oppenheimer analyst Yal Reiner, Apple started warning smartphone makers such as Motorola and HTC in January. It clearly stated that it was not happy to see allegedly iPhone-related IP showing up in their new products.

The Palm Pre also openly flaunted multitouch capabilities, but its lower sales haven't attracted Apple’s attention.

Apple has also taken the matter to the International Trade Commission, seeking damages and an order prohibiting the sale of the devices in the US.

Microsoft is reportedly keen to join battle with HTC and other smartphone makers facing legal action taken by Apple.