London — The consequences in defense terms of electing a British Labour government have been spelled out in a new policy document already approved by the opposition party's executive.
The paper, which has the enthusiastic support of Labour's new leader, Neil Kinnock, advocates a non-nuclear defense policy for Britain and forecasts scrapping the Trident strategic program, decommissioning the existing Polaris system, and removing American military bases from Britain.
The fruit of months of study by a committee that included left- and right-wing members of the party, the new policy has been attacked as a ''cowards' charter'' by Prime Minister Margaret Thatcher's Conservatives.
Publication of the paper represents a determined attempt by Mr. Kinnock to end internal party squabbling over defense - an issue on which Labour appeared confused and self-contradictory at the general election in June last year. The Conservatives won that election in a landslide.
The one aspect of the Labour plan that will please midstream Labour supporters is its advocacy of Britain remaining in NATO. But it should do so, the paper says, only as a nonnuclear member, with renegotiated defense links with the United States.
The defense policy document rests on three arguments:
* British and NATO defense should aim at deterring Soviet aggression.
* The West should try to reassure the Soviets that it is not searching aggressively for military superiority.
* Nuclear weapons are an unstable and self-destructive form of defense, and Britain should renounce them.
In the 1983 election, Labour asked voters to support a defense policy that would cancel the Trident nuclear program but probably retain the Polaris system. Many rejected the policy as lacking in credibility.
To end backbiting in party ranks, Mr. Kinnock and the Labour leadership have opted for a set of policies which, though controversial, at least have the hallmark of intellectual consistency.
The policy document will be refined in the next year or so as the Labour Party prepares for the next general election. Already it is obvious that if Labour comes to power, a serious crisis on defense matters with the United States is probable.
Labour would ask US servicemen at nuclear and non-nuclear bases to leave the country. It would also have to ask the US and NATO as a whole to hold negotiations with Britain about the latter's withdrawal from the West's nuclear defense planning arrangements.
The wording of the policy document is uncompromising: ''Membership of NATO does not require member states to have nuclear weapons of their own or have US nuclear weapons on their territory, or to accede to a strategy based on nuclear weapons.''
Kinnock has pledged himself to support a policy line that would give Britain a leading role in advocating establishment of a nonnuclear defense for NATO.
The document declares: ''It is time for a new internationalist initiative to transform relationships within NATO and the Warsaw Pact which could assist the phasing out of the cold war bloc politics into which Europe is currently frozen.''
The Thatcher government, while condemning the Labour policy document as dangerous and cowardly, is inclined to accept that if the opposition sticks to its new defense approach, voters at the next election will have a much clearer choice than at the last.
That choice will be between a Britain locked into NATO's broad nuclear defense strategy and closely allied to the US, and a Britain determined to renounce nuclear weapons and distance itself from US military support, nuclear or conventional.