Despite selling more than one million Kindles a week during the Christmas period, Amazon's Kindle range did not affect sales of the iPad, which saw a sales increase of 111 percent.

During a conference call to investors about Q1 earnings, Apple CEO Tim Cook said that the impressive Kindle sales did not dent the iPad's success and the Apple tablet's ecosystem is "in a class by itself."

"I looked at the data - particularly in the US - on a weekly basis after Amazon launched the Kindle Fire," Cook said. "And in my view there wasn't an obvious effect on the numbers [of iPads sold]"

The iPad returned record-breaking sales for Apple, with more than 15 million iPads sold over the last quarter representing a 111 percent increase on the year-ago results.

Cook referred to competing tablets and e-readers as "limited-function", adding that he does not think customers who want an iPad will settle for a limited-function device.

"2011 was meant to be the year for the tablet, but most would agree that it was the year for iPad for the second year in a row," Cook added.

Accepting that there has been cannibalisation of Mac sales by the iPad - where customers have chosen to buy an iPad instead of a Mac - Cook added: "I clearly believe, and many others in the company believe, that there will come a day when the tablet market, in units, is larger than the PC market."

Finally, when asked by an analyst if he thought the tablet market was a two horse race, the Apple CEO admitted: "There's a horse in Redmond [home of Microsoft] that always suits up and always runs, and will keep running...we just want to stay ahead and be the lead one."