Nokia's shares, which have halved in price since February as the handset maker fights to restore its fortunes and reverse market share losses to nimbler rivals, fell below 4 euros and to their lowest levels since January, 1998.
Nokia Siemens Networks (NSN), the world's No. 2 maker of wireless networking gear, has struggled to make a profit since its formation in 2007, hit by falling operator spending and price cuts by market leader Ericsson
Both parents have written down the value of their holdings in NSN, the equity of which Bernstein Research has valued at 7-8 billion euros ($10 billion to $11.4 billion)
Analysts said a private equity offer for NSN -- which Nokia and Siemens turned down -- likely valued the venture well below that.
During 12 months of talks with private equity on a sale, NSN's operations have improved, helped by a boom in new network gear.
This seems like a sensible decision from parents given that Nokia Siemens Networks is starting to turn around its fortunes, said analyst Paolo Pescatore from research firm CCS Insight.
The venture is expected to report an underlying operating profit of 75 million euros ($107.3 million) -- larger than Nokia's phone unit -- on sales of 3.4 billion for the April-June quarter when it reports on July 21.
NSN said it plans to further improve the competitiveness of the company as a standalone entity. It has cut thousands of jobs since it started operations in 2007.
In a statement both parents reaffirmed their commitment to the venture but a spokesman for Siemens said the German company was still looking to exit the venture sooner or later.
Siemens has been looking for an exit since Peter Loescher took over as group chief executive shortly after the venture started operations, sources have told Reuters.
While telecom gear has been much closer to Nokia's core business over the years, its focus has shifted increasingly to its struggling cellphone business.
Nokia has on several occasions said that it (NSN) is not its core business and probably they will continue to find some kind of solution to this, Pohjola Bank analyst Hannu Rauhala said.
Shares in Nokia were 3.4 percent lower at 4.04 euros by 1437 GMT, after hitting 3.99 euros earlier, while Siemens shares were 1.9 percent higher at 94.78 euros.
The timing for selling the unit could hardly have been worse; there are clear signs from companies like LG and ASML of European handset market descending into a slump and selling a mobile network company on the eve of a handset market downturn is a tough challenge, said MKM Partners' analyst Tero Kuittinen.
Nokia's share price slump follows fears that the company's shift to Microsoft
At the end of May Nokia warned that its second-quarter results would be well below a previous outlook and ditched its full-year targets.
(Additional reporting by Jens Hack in Munich; Editing by David Holmes and David Cowell)