Apple may not be revealing exactly how many watches its has sold in the last quarter, but analysts at research firm Juniper have been crunching the numbers and come up with an estimate of up to 2.5 million.

The company said the Apple Watch could have brought in as much as $1 billion in revenue for the technology company over the three month period, and expects sales to reach more than 6 million by the end of the year. If Juniper’s figures are accurate, that would make Apple’s device the best selling smartwatch in the market.

Revenue from the Watch was referenced in Apple's quarterly results under the "other products" category. Juniper based its figures on statements from the company's executives, including chief executive Tim Cook who said "It would not be an inaccurate thing to look at the sequential revenue, the year-over-year change and assume that was the total watch revenue".

Until the Watch launched, revenue in this sector was declining at a rate of about $200 million per quarter, as sales of iPads and other accessories declined.

Apple CFO Luca Maestri also had something to add on the matter, saying Apple Watch's contribution "accounted for well over 100 per cent of the growth of the category". He also the Watch sold more than the first iPhone or the iPad in a comparable period; the first iPad sold 2 million in 60 days.

"However, despite buyers being satisfied with the device, we continue to see that the product is running out of steam. We would also expect a decline in sales following launch, especially as H1 draws to a close," said Juniper devices analyst James Moar. "This means we are unlikely to see more than 7 million Apple Watches sold by the end of 2015, even with a boost to sales over the Christmas period."