In opening arguments on Monday in the court fight between the Federal Trade Commission and Staples, Tara Reinhart argued for the FTC that Staples' $6.3 billion bid for Office Depot is illegal because the two companies sell 79 percent of the pens, paper, file folders and other "consumable office supplies" sold to Fortune 100 companies.

She showed emails, with company names blacked out, from firms that expressed concern that the proposed merger would mean higher prices for them. Staples announced its proposed acquisition of Office Depot in February 2015. [nL4N0VE540]

Reinhart argued that Amazon is not a true competitor to Staples or Office Depot and said not a single large business customer has contracted with Amazon as its primary supplier.

However, Staples' lawyer, Diane Sullivan, argued that her client was "like penguins on a melting iceberg," referring to Amazon's 2015 announcement that it planned to enter the office supplies market in a serious way.

Sullivan also said the FTC should take note of tough Internet competition that helped force retailers including Circuit City Stores Inc [CCTYS.UL], Borders Group Inc [BDGUP.UL] and Radio Shack to seek bankruptcy protection.

"The landscape of history is littered with companies who have been killed or bankrupt by digitized companies like Amazon," she said.

The hearing is expected to last for up to two weeks. To win a preliminary injunction, the U.S. government must prove that the proposed acquisition will hurt consumers in the form of higher prices or lost innovation.

Such a finding would halt the deal long enough for the FTC to hold hearings in its internal court, an extremely slow process. A preliminary injunction often leads to companies deciding to abandon their deal.

Judge Emmet Sullivan said it was "troubling" that granting a preliminary injunction could kill the deal before it goes to a full trial. "In reality, investors aren't going to wait around," he said.

By Diane Bartz and Clarece Polke

Stocks treated in this article : Staples, Inc., Amazon.com, Inc., Office Depot Inc