WARSAW, Poland — German novelist Günter Grass said in a letter to the mayor of his hometown of Gdansk that only in his old age has he found the “right formula” to talk about having served in the Waffen-SS during World War II.

“In the years and decades after the war, when the terrible scope of Waffen-SS crimes was revealed, I kept to myself this episode from my young years that was brief, but which weighed on me heavily,” Mr. Grass wrote in the letter dated August 20 and made public yesterday. “However, I did not erase it from my memory.”

“Only now, with age, I have found the right formula to talk about it in a wider perspective.”

Mayor Pawel Adamowicz had the letter read out by actor Jan Kiszkis at a news conference in Gdansk.

Earlier this month, Mr. Grass, 78, made the surprising confession that he served in the Waffen-SS, the combat arm of the Nazis’ fanatical organization. His new memoir, “Peeling the Onion,” was then released and appeared last week in German bookstores and quickly became a best-seller.

Mr. Adamowicz had written to Mr. Grass asking for an explanation amid calls from some politicians to strip the author of his honorary citizenship in Gdansk.

In his letter, Mr. Grass said his book tells how in 1942, as a “blinded 15-year-old, I asked to serve on the submarines, but I was refused. Instead, in September 1944, at the age of 17 — without my participation — I was made a member of the Waffen-SS. I would like to keep the right to say that I have understood this painful lesson that life taught me when I was a young man. My books and my political activity are the proof,” Mr. Grass wrote.

“This silence may be judged as a mistake — that’s exactly what’s happening. It may also be condemned. I must also come to terms with the fact that the honorary citizenship of Gdansk is questioned by many residents.”

But Mr. Grass did not say he was giving up his honorary citizenship, as he has been urged to do by Solidarity founder and Nobel Peace laureate Lech Walesa.

Mr. Walesa had threatened to give up his own honorary citizenship in Gdansk if Mr. Grass did not give an explanation to the city. But he said he was satisfied by the letter and would not do so now.

“I find it a convincing letter, and, from now on, I will no longer be in conflict with Mr. Grass. I think he has explained himself well enough,” Mr. Walesa was quoted as saying by the news agency PAP.

Mr. Grass, the acclaimed author of the classic “The Tin Drum” and many other writings, served in the 10th SS Panzer Division, which fought Soviet troops in eastern Germany near the end of the war. He was wounded and then taken prisoner by American forces.

Mr. Grass, who won the 1999 Nobel Prize for literature, has long been respected as a moral authority in Poland and elsewhere. The Nazis subjected Poland to a brutal invasion and occupation, and Poles enthusiastically welcomed the fact that Mr. Grass for decades urged his fellow Germans to confront their nation’s past crimes.

Mr. Grass was born in Gdansk in 1927 when it was called Danzig. The Baltic port city, now in Poland, passed between German and Polish rule for centuries.