Apple's 10 September iPhone event is just around the corner, but that hasn't stopped speculation about the iPhone 6 that is expected to arrive next year.

The Wall Street Journal reports that, as well as preparing to launch the iPhone 5S and iPhone 5C at its event next week, the company has also been busy working on a bigger iPhone that could arrive in 2014.

Above: The invitation for Apple's iPhone event next week

According to WSJ's sources, Apple has begun testing iPhones with screens ranging from 4.8in to 6in, which would make the new iPhone significantly bigger than the iPhone 5's 4in screen.

Smartphones from rivals such as Samsung have been growing in size, with the screen of the Galaxy S4 measuring 5 inches diagonally and the newly unveiled Galaxy Note 3 'phablet' sporting a 5.7in display. The report that bigger iPhones are in the pipeline suggests Apple is hoping to compete directly with Samsung's large-screened devices.

"People familiar with the company's internal deliberations and plans indicate it appears more willing to move ahead than in years past," says WSJ's report, which adds that Apple seems most keen on the 4.8in screens it has been testing.

It's widely expected that the iPhone 5S and iPhone 5C models that are rumoured to be launching on Tuesday will boast the same 4in Retina display as the iPhone 5, so a bigger model isn't believed to be on the cards until next year.

WSJ notes that it is unclear whether Apple will decide to stick with one bigger screen size or launch multiple iPhones with varying screen sizes up to 6in. Of course, should Apple decide that it doesn't like the bigger screen sizes following its tests, it may choose not to launch a bigger iPhone at all.

In April, Apple CEO Tim Cook said that the company wouldn't launch a bigger iPhone until building one doesn't require any 'trade-offs' in order to accommodate a larger display.

Apple's sent out invitations for its 10 September event earlier this week, and we expect it to be an exciting one.

See:

What to expect from Apple's 10 September event

iOS 7 preview