(Corrects to show Lazaridis now controls, rather than holds direct ownership of, 8 percent of the company's shares in the second paragraph)
By Alastair Sharp
TORONTO, Oct 10 (Reuters) - BlackBerry Ltd co-founder Mike Lazaridis has increased his stake in the struggling smartphone maker and is considering buying the entire company, according to a securities filing on Thursday.
Lazaridis, who now controls 8 percent of the company, has engaged Goldman Sachs and Centerview Partners LLC to assist with a strategic review of his stake. He held almost 5.7 percent as of the end of 2012, according to Thomson Reuters data.
The filing said that Lazaridis, who stepped down as co-chief executive and co-chairman of the company in early 2012, made the move along with his fellow co-founder Douglas Fregin.
The pair, who founded the company then known as Research In Motion in 1985, earlier this year started Quantum Valley Investments to support quantum physics and computing breakthroughs.
Such a deal could be an alternative to a preliminary, $9-a- share offer by a group led by BlackBerry's biggest shareholder, Canada's Fairfax Financial Holdings Ltd.
BlackBerry put itself up for sale in August after lackluster sales for its new devices.
Sources close to the matter have told Reuters that BlackBerry is in talks with Cisco Systems Inc, Google Inc and Germany's SAP AG among others, about selling them all, or parts of itself. The potential buyers have declined to comment.
Blackberry shares turned positive after the news and were trading at $8.20 on the Nasdaq. The stock has fallen more than 20 percent since the company announced a deep loss and sharp fall in revenue in mid-September. (Additional reporting by Solarina Ho; Editing by Jeffrey Hodgson and Bernard Orr)