UPDATE: A Gfinity spokesperson has contacted VideoGamer.com to clarify that the 18 million figure quoted by Upton is not, in fact, an official sales figure provided by Microsoft, but a figure "based on a variety of independent sources".
"Gfinity would like to clarify that it has not sought to confirm Microsoft sales figures," the spokesperson said in a statement provided to us. "The comments in the statement today is based on a variety of independent sources published earlier this year."
As such, it remains unclear exactly how many Xbox One consoles Microsoft has sold.
ORIGINAL STORY: Xbox One has sold 18 million units worldwide, according to information revealed by the CEO of eSports company Gfinity.
Discussing the launch of the company's new Tournament Build App on Xbox One, CEO Neville Upton revealed that the app would allow Gfinity "to reach Xbox One's 18 million users globally, offering this new larger audience the opportunity to interact with the Gfinity brand."
The figure puts Xbox One significantly behind PS4, which had sold 35.9m units as of January 3, 2016.
Microsoft has not officially announced sales figures since November 2014, when it revealed that Xbox One was approaching 10 million units shipped.
Gfinity's figure tallies with that hinted at by EA CFO Blake Jorgensen earlier this year, who estimated that PS4 and Xbox One had sold around 55 million units between them, putting Xbox One at around 19 million.
Microsoft is currently rumoured to be working on a successor to the Xbox One, with speculation pointing towards a reveal at E3 this June. Yesterday, Xbox chief Phil Spencer revealed that the company intends to talk about its plans for Xbox One, Windows 10 "and beyond" in the weeks and months ahead.