Microsoft has said it will miss its target of getting its Windows 10 operating system running on more than one billion devices by 2018.
In a statement, it said problems with its smartphone business would delay it hitting the milestone.
The software giant has struggled to find customers for Windows phones in a market dominated by Android and Apple.
Microsoft says Windows 10 is in active use on about 350 million separate devices at present.
Released in July 2015, Windows 10 has been built to act as a unifying platform across desktops, laptops, tablets, smartphones, small embedded systems and the Xbox One games console.
Shortly before the main release of the operating system, Microsoft set itself the target of getting the software on to one billion devices.
However, its plans took a knock soon after when it sacked 7,800 people in its smartphone business and wrote off the entire value of the Nokia phone business it bought in 2014 for $7.2bn (£5.4bn).
Earlier this month, it scrapped its Finland phone unit and laid off more staff.
At the time it made the prediction about Windows 10 use, Microsoft was hoping to sell about 50 million Windows phones a year.
However, global smartphone sales figures from Kantar Worldpanel suggest Windows phones now account for only 1.6% of sales in North America and 0.4% in China - the two biggest markets for smartphones.
Microsoft said the "focusing" of its smartphone business would mean it would now take longer than three years to hit the one billion figure.
It said it expected adoption numbers to get a boost from businesses starting to use it and because it will be used on new devices Microsoft and its partners are planning.
However, as Peter Bright at Ars Technica pointed out, demand for Windows 10 looks set to slow in the near future thanks to dwindling sales of PCs and because the Microsoft programme that lets people upgrade to the OS for free will stop at the end of this month.
Many people have also been reluctant to install the software on their home computers because of the "aggressive" tactics Microsoft has used to prompt them to update.