Staples Inc. said the U.S. Federal Trade Commission's opposition to its $6.3 billion acquisition of Office Depot Inc. was "simply wrong" and that it did not take into account increasing competition from online retailers.

The FTC filed a lawsuit in December to block the deal. Hearing on the lawsuit starts on Monday in the U.S. District Court for the District of Columbia.

The FTC has said a merger between the two could reduce competition for nationwide contracts for office supplies.

It has also rejected Staples' raised offer to divest up to $1.25 billion of commercial contracts to secure approval for the deal.

The companies on Friday reiterated that the FTC's lawsuit was based on a flawed analysis of the office supplies marketplace and a deep misunderstanding of the competitive landscape.

They said the FTC had refused to acknowledge the rise of new competitors, such as Amazon.com Inc, and the disruptive effects of the digital economy.

"This has been a long and frustrating road, but we look forward to a fair and impartial hearing," the companies said in a letter to customers.