NATO, in talks with Moscow and its Warsaw Pact allies, should negotiate a defensive posture in Europe that would leave both sides incapable of threatening each other, a panel of arms control experts said Friday.

"The idea of a war-free Europe is not a fantasy," said the group's report, released in Washington, Bonn, Brussels and London."There is a window of opportunity now to begin the process of dismantling the military confrontation in Europe. There is a danger, if the (talks in Vienna) drag on year after year, that the chance will be lost," the report said.

The document was prepared by the British American Security Information Council, an independent research group in London and Washington; the Alternative Security Working Group of London; and the Committee for National Security based in Washington.

In Washington, the report was to be detailed Friday by former CIA Director William Colby and Rear Adm. Elmar Schmahling, chief of the Office for Studies and Exercises, a West German military think tank.

The three groups said they decided to produce their own suggestions for European arms control because NATO has not proposed a comprehensive concept for arms control it promised two years ago.

At the moment, NATO and Warsaw Pact forces are grouped against each other on the border of East Germany and West Germany.

The groups suggested that NATO and the Warsaw Pact shift to the concept of "mutual defensive superiority." NATO now uses a "flexible response" doctrine that does not rule out the first use of tactical nuclear weapons as well as the forward deployment of forces.

"If the two alliances can restructure their forces so that they are strong in defensive capabilities and weak in offensive ones, then a successful offense could not be launched," said the group's report.

The experts suggested negotiators consider "a defensive zone along the border from which all major categories of offensive weapons are excluded, but where full defensive preparations are permitted."