The credit crisis could dump nearly 6 million square feet of top-tier, Midtown office space into the market over the next two years, putting downward pressure on rents, a new report shows.

Much of that inventory would come from chunks of empty office space totaling 100,000 square feet or more, whose availability since July 2007 has surged 42 percent, according to the report by broker Newmark Knight Frank.

“A year ago, there were 10 spaces [in that category] available and now there are 50,” said Ray Sandeverino, chair of real estate at law firm Loeb & Loeb, which has been putting off its own search for 150,000 feet while waiting for rents to drop.

Newmark estimates 1.845 million square feet could become available soon, and predicted another 4 million could be vacated between now and 2010 as financial firms shutter, downsize or consolidate.