Joseph Rotblat, 96, a British physicist who received the Nobel Peace Prize in 1995 for his decades-long campaign to end the nuclear arms race, died of a heart ailment Aug. 31 at a hospital in London. He once said he was among those "who have been trying for 40 years to save the world, sometimes against the world's wishes."

Dr. Rotblat had a promising early career in nuclear physics and was recruited to the Manhattan Project, the Allies' World War II atomic bomb program. He later quit on moral grounds when intelligence reached him that the Nazis' bomb effort had sputtered, which in his mind made the U.S. weapon unnecessary.

He had also been appalled when he overheard Leslie R. Groves, the American general leading the project, say the creation of the bomb would "subdue the Soviets" after the war. This fostered his lifelong suspicion of the U.S. military and the role scientists should have in developing weapons.

Starting in the late 1940s, Dr. Rotblat joined philosopher Bertrand Russell and physicist Albert Einstein in advocating peaceful use of new arms technologies. He was a signer of the Russell-Einstein Manifesto of 1955, which warned that nuclear weapons would "threaten the continued existence of mankind" and urged diplomacy among nations.

The manifesto brought Dr. Rotblat, regarded as a pithy and passionate lecturer, a foundation from which to organize what became the Pugwash Conferences on Science and World Affairs. With seed money from American industrialist Cyrus S. Eaton, Dr. Rotblat established the Pugwash conferences in 1957 to unite scientists as private citizens and not as representatives of their countries.

The gatherings, named after the fishing village in Nova Scotia where the first was held, were a brave idea during the Cold War.

"Anyone in the West who came to a meeting, who talked peace with the Russians, was condemned as a Communist dupe," Dr. Rotblat later said. Participants included Soviet physicist Andrei Sakharov and American scientist Frank von Hippel.

Gradually, Pugwash became a way to maintain scientific back-channel communications between Western and Iron Curtain countries. The small, invitation-only conferences provided a discreet atmosphere to discuss nonproliferation treaties and bans on weapons testing.

Pugwash, which shared the Nobel Prize with Dr. Rotblat, held great symbolic appeal. The Nobel committee seemed to acknowledge as much when, at the time of the ceremony, it criticized France and China for conducting nuclear weapons tests despite international opposition.

Dr. Rotblat was born in Warsaw on Nov. 4, 1908. His prosperous Jewish family, which was in the paper business, saw its finances collapse during World War I. To survive, family members distilled vodka in their basement.

Dr. Rotblat worked as an electrician while taking night classes at the Free University of Poland, where he received a master's degree in physics in 1932. He received a doctorate in physics from the University of Warsaw in 1938.

He accepted an offer at the University of Liverpool to work under physicist James Chadwick, who had won the Nobel Prize for his discovery of the neutron. "In Liverpool, they were building a cyclotron," he told the Bulletin of the Atomic Scientists. "It was my intention to build a cyclotron when I came back to Warsaw so that we could start a proper school of nuclear physics."

Nuclear fission was a new and exciting field at the time, and he immediately understood the implications if Germany developed an atomic weapon first. He later called this the classic argument of deterrence.

The race was personal as well as patriotic. His wife, the former Tola Gryn, had stayed in Poland for financial and health reasons. They lost contact in 1940, after the Nazis invaded, and he never heard from her again. He leaves no immediate survivors.

Dr. Rotblat spent less than a year in Los Alamos, N.M., where he worked on the Manhattan Project, but it was enough to ask to resign. This was an unusual step that Chadwick frowned upon, and security officials questioned it as a loyalty matter.

After the atom bomb blasts over Japan, he saw little reason to stay mum about his secret work. With Einstein and Russell, he founded the Atomic Scientists Association as a forum to denounce nuclear weapons.

In his Nobel acceptance speech, Dr. Rotblat gave scientists great credit for political influence and held in disdain the "ivory tower mentality," which he said made some think that "science is neutral" and that "science has nothing to do with politics."

He said he was not asking people to relinquish their loyalty to their countries but instead to "extend our loyalty to the whole of the human race."

Dr. Rotblat spent the rest of his career at St. Bartholomew's Hospital, at the medical college of the University of London. He specialized in studying radiation's effects on living organisms.

He wrote more than a dozen books, among them "Radioactivity and Radioactive Substances" (1953), which was co-authored by Chadwick, and "War No More: Eliminating Conflict in the Nuclear Age" (2003), with behavioral scientist Robert Hinde.

Joseph Rotblat won the Nobel Peace Prize in 1995 for efforts to end the nuclear arms race through the Pugwash conferences.