John Hancock Financial Services, one of the pioneer corporate presences in Boston’s Seaport District, will consolidate its Boston offices in Back Bay and leave behind its 465,081-square-foot headquarters at 601 Congress St. by the end of the year.

The move presents an potential short-term office availability in the sought-after Seaport. Few large blocks of space are available following headquarters leases last year by Alexion Pharmaceuticals, educational publisher Cengage and Needham tech company PTC at spec office buildings under construction at Pier 4 and 121 Seaport.

Approximately 1,100 John Hancock employees will move to 200 Berkeley St. and 197 Clarendon St. by the end of 2018. The connected buildings total 1.2 million square feet.

“We chose the Back Bay campus because it offers us the most square footage to be together in a space right-sized for our workforce,” CEO Marianne Harrison wrote in a memo to employees. “One of the first things I noticed when I started in October was how much physical room we have for a team that doesn’t remain seated at a desk all day, with many employees in roles that enable them to take advantage of our WorkSmart flexible work program.”

Hancock received city approval in 2015 for a 26-story, 625,000-square-foot office tower at 380 Stuart St. to replace its existing 9-story office building. Development plans for that property are continuing, the memo states.

The financial giant developed the 601 Congress St. offices in 2004 as one of the first major employers to put down stakes in the Seaport District. No decision has been made whether to sell or lease the distinctive 14-story mid-rise, John Hancock spokeswoman Anne McNally said. The property is currently assessed at $254.5 million.

Vacancies briefly shot up in Back Bay following the departure of large office tenants including Houghton-Mifflin Harcourt and Wells Fargo, but have dipped following big leases by DraftKings and WeWork, while Wayfair is on the hunt for a big expansion in the neighborhood as well.