Rumours that Saab have donated funds to the "yes" campaign have been met with dismay by both sides of the debate, news agency TT reports.
"It doesn't exactly help", MP Thomas Hurter from the "Yes" camp says, "it's very unusual that a company pays money straight into a referendum campaign. If they want to support an issue they usually do it via an organisation."
Saab have neither confirmed or denied the story, which has led to many Swiss commentators concluding that the rumours are true.
Social Democrat MP Evi Allemann says it is problematic that a foreign company is getting involved in a national decision. "We have no clear rules for referendum campaigns", she told TT, "so foreign companies can shamelessly get involved in national decisions. It's hard to say what the impact of Saab's involvement will be, but I think it will be counterproductive."
The Swiss government is planning to buy 22 Jas Gripen planes at a price of around SEK 23 billion, but it first has to be approved by a national referendum.