Apple has withheld billions of dollars in payments to chipmaker Qualcomm, taking what was a contract dispute into what one analyst called an “all-out war”.

Qualcomm is one of the main suppliers of components to the iPhone, its most important product. Since January the feud has helped drive down its share price by more than 15 percent.

Apple has claimed that Qualcomm is using a monopoly position in cellular chips to impose “onerous, unreasonable and costly” terms on customers and competitors.

Qualcomm now says it expects revenue to be eight percent to nine percent below its previous forecast.