Borrowers will soon wave the white flag on a sizable percentage of office buildings across the country, two major CEOs in the sector say.

Of the $1.5T worth of office loans coming due in the next three years, a third will either be foreclosed on or returned as deeds in lieu of foreclosure, PGIM CEO David Hunt told CNBC on Wednesday. Cantor Fitzgerald CEO Howard Lutnick predicted in an interview with Bloomberg that 20% of the U.S. office market will "have the keys handed in."

Tenants' flight to quality has left 60% of U.S. office inventory in purgatory, Hunt told Bloomberg. Buildings losing value with maturing loans are facing capital markets that aren't just more expensive for borrowing — many lenders simply won't originate new office loans at all.

As recently as the first quarter, many market observers held out hope that the Federal Reserve would begin to pull back on its benchmark interest rate by the second half of this year. Instead, the Fed keeps hiking.

"Let's face it, if rates go to 8% or 9% and your rents haven't gone up like that, the math doesn't work," Lutnick told CNBC. "Give the keys back to the bank."

As the broader market has settled into the reality that hybrid work schedules aren't going anywhere, the largest office users of the past have been shrinking offices when their leases end and putting massive chunks of space up for sublease. For office buildings built even 15 years ago, it would take a large expenditure to compete for what little leasing business is left, Hunt said.

As cash flow decreases and inflation keeps operating costs high, borrowers with big enough portfolios to absorb the damage that a deed-in-lieu transaction can wreak on a lender relationship have already begun to walk away from properties, as RXR did with 61 Broadway in New York's Financial District.

CORRECTION, JUN. 8, 5:10 P.M. ET: A previous version misstated Hunt and Lutnick's employers. This article has been updated.