United Mine Workers members and their families will have all medical and death benefits cut off if the union strikes the coal industry as expected on Dec. 6, UMW President Arnold Miller announced yesterday.

Pensions of retired miners also will have to be reduced starting in January if there is a strike, Miller said.

Such a disclosure of weakness by a union leader at the height of contract bargaining is unusual.But Miller associates indicated he felt he had no choice but to prepare members for the consequences of a strike, even if the action could undercut the union's bargaining power, already weakened by huge coal stockpiles and an increasing supply of non-union coal.

If benefits are cut off, it would be the first time since the industry financed benefits program went broke in 1949 that it halted all payments to its recipients, now numbering 815,000 for health benefits and 80,000 for pensions.

A legacy of the late John L. Lewis, the program was once considered a model for the nation's trade unions and was largely responsible for bringing health care to the Appalachian coal fields.

"A curtailment of funds benefits would be a heavy blow to our members and their families," said Miller in a bluntly worded statement conveying a decision made earlier by trustees of the funds, which are administered jointly by the UMW and the Bituminous Coal Operators Association.

Both medical and pension benefits - amounting to about $540 million a year under normal circumstances - are financed by employer contributions that are based on the volume of coal produced and the number of hours worked by UMW miners. The funds dry up when the miners go on strike.

Medical benefits were reduced during a wave of wildcat strikes this summer, forcing miners to pay up to $500 a year per family for services that were previously free. Further cutbacks were only narrowly averted by an end to the 10 week walkout in late August, but the funds were unable to build up enough to serve to withstand another production break.

Hence a strike was expected to produce a further curtailment, although abrupt total cutoff was not universally anticipated.

Miller had been accused of withholding disclosure of the earlier cutback to avoid repercussions during his re-election campaign last spring. His statement yesterday was viewed as an effort to avoid further intra-union misunderstandiings and prepare members fully for the consequences of a strike. The prospect of a total benefit cutoff could also strengthen Miller's hand if he comes up with a last-minute agreement for ratification by rank-and-file miners, or if he seeks a contract extension, as he indicated Monday be may do if the talks begin to show progress.

Talks currently under way here between the UMW and the BCOA, the industry's bargaining arm, are reportedly moving slowly. While the current contract does not expire until Dec. 6, both sides agree that an agreement would have to be reached by the end of this week to give miners time to ratify it before the strike deadline.

Although a strike is widely anticipated, the prospective benefit loss raises new doubts about how long a strike could be sustained by the union's roughly 270,000 members.

The nation's energy supplies are in much better shape than the union's benefit fund, and reportedly sufficient to withstand a strike of three months or more.

In his statement, Miller denied that the prospect of benefits termination will affect UMW's position at the bargaining table. "It would be a tragic mistake for the BCOA to decide that this action by the trustees puts unbearable pressure on the (union)," he said. "It will not make us knuckle under to the BCOA." The union can take a strike, "a long one if necessary," Miller added.

A funds official said it may be possible to pay health benefits if an "agreement in substance" short of a ratified contract is reached by Dec. 6, but warned that pension benefits also could be eliminated entirely. Miller's statement referred only to pension reductions. Pensions are currently financed from borrowed funds, and there is uncertainty over whether assistance will be forthcoming from the Federal Pension Benefit Guaranty Corporation.

Late yesterday it was announced that Harry Huge, the union's funds trustee, is resigning to become special counsel to the UMW for the negotiations. Union dissidents had repeatedly called for Huge's resignation during previous funds crises, but he had refused to step down.

UMW and BCOA negotiators were scheduled to meet last night, for their first evening session since bargaining began Oct. 6, indicating a final push to reach an agreement by the week's end.