Sales of the Raspberry Pi will surpass that figure this month, said the Raspberry Pi project founder Eben Upton.
The milestone comes as the Raspberry Pi 3 is unveiled featuring a faster 64-bit processor and built-in wi-fi and Bluetooth connections.
“We’re calling it,” said Mr Upton. “We’re the best-selling UK computer ever.”
He said the Pi Foundation initially thought it had won the title last year when sales blew past the total set by Sinclair machines in the 1980s.
However, he said, it then emerged that the Amstrad machine had sold in larger numbers.
Now, even that total has been exceeded.
“The curve keeps trending upwards,” said Mr Upton, adding that sales will get a fillip from the release of the Raspberry Pi 3 on 29 February.
It will go on sale for $35 (£30) and a “few hundred thousand units” will be available on launch day from online stores.
“The two main things that people do with their Pi are use it as a PC replacement or use it as an embedded computer,” Mr Upton told the BBC.
“The Pi 3 is doubling down on both those things rather than going looking for new things to do.”
The updated device has a 64-bit processor onboard that gives the Pi 3 a 50% performance improvement on the Pi 2.
Also built in to the new device are wi-fi and Bluetooth connections.
Hobbyists using the Pi to act as a hub for smart home gadgets would be able to use Bluetooth to link devices and sensors together, he said.
“This is the first Pi you can stick behind your TV and completely forget about,” said Mr Upton.
There were also signs that the Pi was succeeding in achieving one of its founding aims – to make computer science a more popular option at degree level.
Mr Upton said that the Pi’s popularity and other coding projects were boosting interest in the technical subject.
In some institutions, the numbers of people enrolling on computer science courses had recovered from the low points seen in 2008-9, he said.
However, he admitted it could still be be “too early” to see the full effect of the Pi on people who wished to pursue coding or their technical skills, given that the first Pi was only released in 2012.