United Kingdom, Sept. 10 -- Apple has unveiled its first foldable iPhone, entering a smartphone market where rivals including Samsung, Motorola and Google have already offered folding devices for several years.

Called the iPhone Duo, the device starts at $1,999 (£1,476), making it the most expensive iPhone Apple has released.

It will go on sale on October 23.

The Duo is roughly the size of a passport when closed and opens to reveal a 7.6in (19.3cm) display, the largest screen ever fitted to an iPhone.

The device can also be used with an Apple Pencil, bringing a feature previously associated with the iPad to Apple's smartphone range.

Users can split the Duo's display between two apps, or open two windows of the same website, whic...