Finnish phone giant Nokia has taken control of Symbian, the mobile phone software firm it helped set up in 1998 to keep Microsoft at bay.
Nokia is paying UK handheld computer pioneer Psion, another of Symbian's founders, about £130m for its stake.
Once the deal is done, Nokia's share will amount to some 63%, having - with Psion - bought out Motorola last year.
The other phone firms with a stake in Symbian will now have to decide if they can live with formal Nokia control.
Nokia has promised that it will keep Symbian, which now has a leading share of the market for the software which runs advanced mobile phones, at arm's length.
But the company's dominance may still swing Symbian's plans, not least because - despite recent slippage - Nokia still sells one in three of all mobile phones bought worldwide.
Among those still holding minority stakes are Ericsson, the fourth of the original founders, Korea's Samsung, Germany's Siemens and Japan's Matsushita and Sony.
All of them use Symbian's software in their "smartphones", a description which usually implies advanced features such as calendars, e-mail access and games, although several also use either proprietary systems or Microsoft's software.
Microsoft's offering has several high-profile takers, including Motorola and Samsung, as well as smaller companies building phones that mobile operators can rebrand as their own.
But Symbian still has a commanding lead in the smartphone market.
For Psion, however, the consequences of the sale could be more immediate.
The company started out as the inventor of a groundbreaking handheld computer, but recently has pulled in its horns to concentrate on rugged mobile devices and wireless networks.
Its Symbian holding was its single biggest asset - and, say some analysts, the main motive for many people to hold its shares.
By 1130 GMT, Psion stock had dropped 30 pence or 31% to 65.75 pence, its lowest ebb for four months.