Five former Pentagon officials and 11 scientists sent a telegram to the White House yesterday asking President Reagan to ban all weapons systems in space.

"We believe that the testing or deployment of any weapons in space significantly increases the likelihood of warfare on Earth," said the cable, which was written by Dr. Carl Sagan of Cornell University's Laboratory for Planetary Study and Dr. Richard L. Garwin of IBM Research Laboratories in Yorktown Heights, N.Y.

"Once such weapons systems are established in national arsenals, they become very difficult to displace," the cable said. "If space weapons are ever to be banned, this may be close to the last moment in which it can be done."

Among those signing the telegram were retired Adm. Noel Gayler, former director of the National Security Agency; Herbert York, one-time director of the Pentagon's Advanced Research Projects Agency; Lee DuBridge, science adviser to President Nixon, and Franklin Long, former associate director of the Arms Control and Disarmament Agency. Also signing were Iowa State University's Dr. James Van Allen, whose discovery that the Earth is ringed by a belt of radiation bears his name; Columbia University Nobel laureate I.I. Rabi; Wolfgang Panofsky, director of the Stanford University linear accelerator, and William Pickering, former director of the Jet Propulsion Laboratory in California.

"In the quarter century since the first artificial satellite was launched, humans have used their new space-faring capability wisely," the cable said. "But fiction writers and military strategists have romanticized the purported inevitability of warfare in space. We join in urging the U.S. and the Soviet Union to negotiate a treaty to ban weapons of any kind in space."