The iPad’s share of the global tablet market fell by 15 percentage points to just over 50% in the third quarter, market watcher IDC has found, compared to the previous quarter.

Apple’s device made up 50.4% of the 27.8 million tablet shipments worldwide during the quarter, but (since-confirmed) rumours of a forthcoming iPad mini resulted in a decline of both consumer and commercial shipments.

Meanwhile, manufacturers whose tablets are based on Android all gained share. Samsung, Amazon.com, Asus and Lenovo collectively shipped 10.4 million units during the third quarter, up from 7.5 million one year ago.

Samsung’s growth was especially pronounced – its market share has grown 325% to 18.4% in the last year. Sales of Amazon.com’s Android-based Kindle tablets doubled to 9% quarter-on-quarter.

IDC says the while launch of its iPad Mini and a fourth-generation iPad will lead to an improved fourth quarter for Apple, the smaller iPad’s relatively high $329 starting price will allow Android vendors to build up on their third quarter success.

Ryan Reith, programme manager at IDC’s Mobile Device Trackers service, said Microsoft has its work cut out ahead of it if Windows-based devices are to take a slice of the market. “With the recent introduction of a number of Windows 8 and Windows RT tablets, consumers now have a third viable tablet platform from which to choose. Price points are critical in tablets, and Microsoft and its partners will have a tough time winning a share of consumer wallet with price points starting at $500.”

The tablet market grew 45.5% year-on-year in the third quarter and 6.7% over the previous quarter.