Matsushita is the world's fourth largest mobile phone maker with Panasonic among its brand names. It joins the top three mobile makers - Finland's Nokia, Sweden's Ericsson and Motorola of the US - who joined the alliance last year.
The addition of Matsushita to the alliance adds credibility to Symbian's ambition to control the operating software of the next generation of mobile phones and palm-top computers, which will provide consumers with Internet access, video telephony and other services.
Symbian's operating system competes with Microsoft's CE technology based on its Windows desktop software. "This is a global alliance with partners in the US, Japan and Europe," said David Levin, Psion chief executive. "We're not looking for any more investors in the partnership."
Matsushita's stake came from a pro-rata dilution of the interests of the existing partners. That leaves Psion with 28.1 per cent and Ericsson, Nokia and Motorola with 21 per cent each, although the stakes could be reduced if the partners opt for a stock market flotation, which Mr Levin conceeds` is possible.
Nonetheless Psion shares slid 5.2 per cent, or 43p, to 789.5p as London investors, following the US, sold off technology stocks.