Telecom giant Verizon is poised to vacate the Center City office tower built for its predecessor company, Bell Atlantic Corp., where rival Comcast has become the dominant tenant.

By the end of the year, Verizon Communications Inc. plans to leave all its space at the roughly one million-square-foot building formerly called the Bell Atlantic Tower and now known as Three Logan Square, spokesman Michael Murphy said.

The move could clear the way for Comcast Corp. — already the tower's biggest  occupant with about 200,000 square feet under lease — to further expand its footprint there, said Mitch Marcus, a managing director at commercial real estate services firm JLL in Philadelphia.

Former city commerce director James Cuorato said it's "certainly ironic" to see Verizon survived at its landline-era forerunner's tower by Comcast, with which it is locked in competition for internet customers.

"I continue to be amazed at how they continue to expand," Cuorato, now president of the Independence Visitors Center, said of Comcast.

The red granite Bell Atlantic Tower at 1717 Arch St. was built in the early 1990s to house the regional phone-network operator's corporate headquarters, though those operations moved to New York around the time of the company's 1997 merger with NYNEX Corp.

But even after the merged New York-based company — still called Bell Atlantic — combined with GTE Corp. to form Verizon, it maintained a significant Center City office presence.

As recently as 2007, Verizon occupied 28 floors of the 1717 Arch St. building, more than half of its 53 stories of offices, according to a directory of Philadelphia office properties from that year.

But in recent years, Verizon's Center City presence has receded. The company now occupies only about 89,000 square feet in the former Bell Atlantic building, less than a tenth of its office space, brokerage Newmark Grubb Knight Frank said in a report this month.

That space is used by employees of Verizon's information technology, network, finance, and sales departments, Murphy said. Those workers will relocate to Verizon's offices in a former Bell switching station at 900 Race St. before the end of the year, he said.

Comcast, meanwhile, is nearing completion of the 60-story Comcast Technology Center tower, with 1.3 million square feet of office space situated diagonally to the former Bell Atlantic building, in addition to its existing Comcast Center headquarters building with 1.3 million square feet of offices.

It also leases about 200,000 square feet nearby at the 709,000-square-foot Two Logan Square building and occupies about 100,000 square feet at the 1.8 million-square-foot Centre Square office complex near City Hall, JLL's Marcus said.

With Verizon's space soon to be available there, the former Bell Atlantic building's proximity to Comcast's other operations would make it a natural location for the expanding and acquisitive technology and entertainment company to grow its Center City office presence, Marcus said.

Comcast also seems a ways off from having yet another Center City office space built for itself: There has been speculation about a third Comcast tower, but a charter school at the center of its likely development site is in no apparent mood to sell its property.

"With the large footprint currently there today, Three Logan would be a very logical and viable consolidation alternative for Comcast," Marcus said.

Comcast spokeswoman Katie Lubenow said the company is looking at its "overall space needs in Philadelphia and has not made final decisions at this time."

Jerry Sweeney, chief executive of Brandywine Realty Trust, which owns the former Bell Atlantic building, said his company had long been aware of Verizon's plans to vacate the tower, and that "several key prospects" have been lined up for the space.