Julia Chapman, one of Mayor Nutter's longest-serving and most trusted advisers, resigned yesterday, effective immediately.

Since 2008, Chapman had served as Nutter's legislative director, making her the mayor's chief liaison to City Council and the legislature. Chapman was an influential force within the administration, known for occasionally clashing with other senior members of Nutter's team.

Chapman was among the few aides who worked for the mayor before his election. For 11 years, she served as chief of staff of his City Council office.

"It's not only a personnel loss, it's a personal loss," Nutter said of Chapman's resignation.

The timing of the announcement - on a Friday evening - and the sometimes contentious relationship between Council and the Nutter administration led some in City Hall to suspect that Chapman's departure was not entirely voluntary.

Nutter denied that. He said Chapman, whose yearly salary was $137,000, had never planned to stay for more than two budget cycles.

The mayor effusively praised her work.

"She knows her stuff, and she's very good," Nutter said. "She's a fighter. She will never give up on trying to get something done."

Chapman did not return a call seeking comment.

The news release announcing her departure did not say what she planned to do next.

City Council President Anna C. Verna was surprised to learn of Chapman's resignation.

"I have known Julia during the many years she spent as chief of staff to Michael Nutter when he was in Council and admired her tremendous work ethic. Her loyalty and intellect continued to be great assets since his election as mayor," Verna said through a spokesman.

Nutter said Chapman's departure was an isolated case of a staffer deciding the time had come to try something else, and not a prelude to a broader shuffling of positions. He said he planned to name a successor in the "very near future."