Although Slice Intelligence's figures have been widely reported, its methodology—which is based on analyzing email receipts from its panel of 2 million online shoppers, according to its website—has been criticized.

Online sales in the U.S. slipped around 90 percent to below 5,000 per day in late June and again in early July, having peaked above 40,000 in mid-to-late April, according to Slice Intelligence.

"Apple Watch sales continue to decline in June," Slice Intelligence tweeted on Tuesday, alongside a graph showing a steep decline in demand for the smartwatch since its launch date.

Online sales of the Apple Watch in the U.S. have fallen by 90 percent since the device launched in April, according to market researcher company Slice Intelligence.

In addition, Annette Zimmerman, an analyst with research company Gartner, said that the U.S. sales "do not tell the full story."

"This company only counts online order confirmations in the U.S.; they can't measure anything outside the U.S.," she told CNBC via email on Wednesday. "There are other big economies like Germany and Japan where the watch is selling. The trend curve is probably similar (strong sales in the first weeks and then slowing after) there, but the U.S. sales do not tell the full story."

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She was positive about the Apple Watch's sales prospects.

"Gartner have a forecast for the total smartwatch volume for 2015 and that is 40 million and in this 40 million our current assumption is that Apple could do 15-20 million units in 2015," she said.

"I am still optimistic that Apple will get high volumes for the Apple Watch during Christmas."

An Apple spokesperson could not provide sales figures for the watch on Wednesday, but repeated what Apple's CEO Tim Cook said in his last earnings call, that "right now, the demand is greater than the supply".

If there has been a fall a sales of the device, one reason could be rumors about the next generation of the Apple Watch. In June, S&P Capital IQ analyst, Angelo Zino, told CNBC that consumers could be holding out for an upgraded version of the device.

He forecast that a second version of the watch would launch next year.

—With contribution from CNBC's Althea Chang.

Follow us on Twitter: @CNBCWorld

