Jim Balsillie, formerly the largest shareholder and co-chief executive of struggling smartphone company BlackBerry, has sold his entire shareholding in the business.

The move, revealed by a filing released this week, occurred before the end of 2012, weeks ahead of the launch of the new BB10 software and Z10 handset by new chief executive Thorsten Heins in an attempt to reboot the company's fortunes.

Balsillie's shares would have been worth between $167m (£108m) and $396m, depending on when he sold them.

The news is a further buffeting for BlackBerry, which despite proclaiming strong – though unspecified – early sales in the UK for the Z10, has been hit by further defections, including the decision by the US DIY chain Home Depot earlier this week to abandon its handsets in favour of 100,000 10,000 iPhones from Apple.

The company has posted operating losses totalling $1.3bn in its past four financial quarters, and is now in a struggle with Microsoft's Windows Phone to carve out a spot as the "third ecosystem" in the smartphone market that is dominated by Apple and Google.

"Any time a single shareholder with over 5% of the float dumps everything, fellow investors should care," commented Kevin Restivo of the analyst company IDC on Twitter. "A cynic might say Jim Balsillie's dumping of BlackBerry stock around product launch time is a big middle finger to his ex-colleagues" – though Restivo suggested Balsillie might have been seeking to diversify his portfolio.

Balsillie and Mike Lazaridis, the other co-founder, were ousted from their positions as joint chairmen and chief executives in a boardroom coup in January 2012, after shareholders grew increasingly impatient with what they saw as management complacency over the risk posed by Apple and smartphones running Google's Android software.

The decision by the Canadian to dump all his stock during 2012 – consisting of 26.8m shares, equivalent to 5.1% of the stock – suggests he sees little promise in the company.

However, Lazaridis, seen as influential in the development of the technology behind the BlackBerry brand, increased his shareholding during 2012 from 26.8m shares, or 5.1% at the end of 2011 to 29.9m, or 5.7% of the shares at the end of 2012, according to corporate filings.

Shares in BlackBerry, which renamed itself from Research In Motion at the same time as it launched the BB10 software, fell in value from $14.79 each in January 2012 to a low of $6.25 in October 2012, and $11.80 on 31 December. Since then, though, they have recovered to $15.05.

BlackBerry said it did not comment on individual shareholders' ownership.

Correction: this article was amended: Home Depot is replacing 10,000 BlackBerrys, not 100,000.