Apple’s mooted large-screen iPhone would provide a boost for the company in the lucrative US market, according to a research note from RBC Capital Markets.

According to a poll of customers cited by analyst Amit Daryanani and reported by Barron’s, more than one third of upgrading users who do not plan to purchase an iPhone would do so if the company offered a larger-screen device, with a significant portion of customers willing to pay a premium for an Apple phablet.

This indicates that not only would the launch of a big-screen iPhone prevent customers from selecting one of the Android-powered alternatives available from a variety of vendors, but that it could also help Apple protect its margins by being a viable premium product.

It has long been expected that Apple will add to its iPhone line with a large-screen sibling for the iPhone, as demand for such devices has increased in the recent past. Apple has not increased the size of its iPhone screen beyond the 4 inches introduced with the iPhone 5.

Even ahead of the launch of last year’s iPhone 5s, it was reported that the company was working on iPhone devices with 4.7-inch and 5.7-inch displays. Also mooted have been curved glass and sensors which can detect different levels of pressure.

But Bloomberg this week reported that production of the larger-screen iPhone has proved challenging, meaning that a number of efficiency enhancements need to be made before volume can be ramped.

Occasionally accurate publication DigiTimes said this week that Apple has also rejected bodies for the 4.7-inch screen device made by Catcher Technology, due to issues in the areas connecting metal and plastic parts.

Catcher is believed to be the company which manufactures metal cases used in HTC’s One line, and has previously been linked with Samsung.

Other recent speculation is that the larger device will include a camera with optical image stabilisation (OIS), although this feature will be omitted from the standard iPhone.

The anticipated launch date for the next-generation iPhones will be mid-September 2014, twelve months after the launch of iPhone 5s (and 5c). It is not clear if the large-screen device will debut at the same time as the standard smartphone.