WASHINGTON — Representative John Lewis, Democrat of Georgia and an icon of the civil rights movement, announced on Sunday that he had advanced pancreatic cancer, but planned to return to Washington to continue work and begin treatment.

“I have been in some kind of fight — for freedom, equality, basic human rights — for nearly my entire life,” Mr. Lewis, 79, said in a statement. “I have never faced a fight quite like the one I have now.”

Survival rates for pancreatic cancer are grim, and Mr. Lewis said his cancer was Stage 4, the most advanced. Mr. Lewis said that while he was “cleareyed about the prognosis,” doctors had told him that advances in medical treatment would help give him “a fighting chance.”

“I have decided to do what I know to do and do what I have always done: I am going to fight it and keep fighting for the beloved community,” Mr. Lewis said. “We still have many bridges to cross.”