Apple's iPad is expected to slip behind tablets running Google Android this year for the first time, as Apple cedes more of its market share to the competition.

Research company IDC has said iPad shipments are expected to account for 46% of the tablet market in 2013, down from 51% last year.

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By the end of this year, Android tablets are predicted to increase their market share from 42% last year to 49%, giving them a dominant position.

Apple's iPad, with its 9.7-inch screen, has led the tablet market since its launch in 2010, but that started to change in 2012.

The Android slate market was particularly boosted by the success of Google's Nexus 7 and Amazon's Kindle Fire devices, both of which have 7-inch screens, good technical specs, and the Android ecosystem, all for a cheaper price than the iPad.

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Apple responded by launching its own 7.9-inch screen iPad Mini last year.

IDC said that one in every two tablets shipped in the fourth quarter of last year was "below 8 inches in screen size", and it expects smaller tablets to "continue growing in 2013 and beyond".

The tablet market is certainly booming. Global shipments grew to 128.3m units last year, up from 72m in 2011, according to IDC.

Competition in the small screen device space is expected to get even tougher this year, with new market entrants such as Hewlett-Packard's Slate 7 Android device.

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Meanwhile, Apple's main iPad is being challenged by Microsoft's Surface and other Windows devices. IDC expects tablets running Windows 8 to grow their market share from 1% last year to 7.4% by 2017.

But Apple is not just facing competition in the tablet space, as the firm is also beginning to struggle in the smartphone market.

Main rival Samsung is forecast to sell 290m smartphone units this year, an increase of 35% on 2012 figures, according to data from Strategy Analytics.

Apple's iPhone is expected to see slightly lower growth, at 33%, to reach 180m projected sales this year.

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While the iPhone and iPad continue to grow, Samsung in particular has demonstrated an ability to get the right mix of technology, marketing and brand appeal to challenge Apple.

On Thursday, Samsung will unveil the latest device in the Galaxy S smartphone range, arguably the one real challenger to the iPhone.

Apple investors are also started to express concern that Apple is losing its edge in a post-Steve Jobs era, while anecdotal reports suggest some iPhone users are being tempted away to Google's Android.

This has led Jefferies analyst Peter Misek to suggest the previously unthinkable, that Apple's iPhone could fall from grace in the same way as RIM's once-mighty BlackBerry.

He told Reuters that Apple is now in a defensive position against Samsung devices, which could lead to a fast decline.

"Historically when handset makers fall out of favor (eg, the Razr, Blackberry, HTC) they fall faster/further than expected," Misek said.