Back Bay office rents have hit a landmark – prices for Class A space in one of Boston’s toniest neighborhoods is outpacing its Financial District counterparts by 23 percent, according to a report from Richards Barry Joyce & Partners.

Two buildings are battling for supremacy in their respective markets – in the Back Bay, 500 Boylston St. reigns supreme for rents; and in the Financial District, the buck stops at International Place.

The highest rent in the Back Bay comes from the 25th floor at 500 Boylston St., where a company recently signed a deal in the low-$80s per square-foot, said William Motley, executive director for Cushman & Wakefield in Boston. In the Financial District, tenants of developer Don Chiofaro have paid in the low-$70s per square-foot, Motley said.

"Those are the two buildings leading the charge in terms of highest rents, and I would throw the John Hancock Tower in there as well," Motley said. "But they are smaller tenants."

However, celebration over those prices could be premature, as hundreds of thousands of square feet of space is already sitting vacant, and plenty more could be empty in the coming years, potentially dragging down those sky-high rents.

MFS Investment Management moved to 111 Huntington Ave. in December, leaving about 350,000 square feet of space it formerly occupied at Equity Office Properties’ 500 Boylston St. While some of the space has been leased, there is still about 250,000 square feet available, David Richardson, executive vice president with McCall & Almy, told Banker & Tradesman.

Then, State Street’s lease for about 375,000 square feet in the low-rise elevator bank space of the Hancock Tower expires in 2014. The firm plans to move to its new Fort Point headquarters at the end of next year. To date, several firms have toured the space the company is vacating in the Hancock Tower, but none have committed.  

In addition, John Hancock’s Manulife Financial leases about 200,000 square feet at 101 Huntington Ave., about 50,000 is already sub-leased to other tenants. The firm will likely vacate the property entirely and move into space it already occupies on Berkeley Street and in the Seaport, according to industry executives.