Membership of the United Auto Workers union has dropped below half a million for the first time in more than six decades, underlining the wrenching adjustments facing workers once viewed as the titans of the US labour movement.

The UAW disclosed in a regulatory filing on Friday that it had 464,910 members at the end of last year, which was 73,500 or 14.7 per cent fewer than in December 2006 and more than two-thirds below the peak of 1.5m in the late 1970s.

General Motors, Ford Motor, Chrysler and their suppliers, notably Delphi, have shed more than 100,000 workers over the past two years as they have shrunk North American production capacity in line with their eroding market share.

Job losses have been exacerbated by an unexpectedly steep downturn in car and truck sales. All three Detroit carmakers are currently in the throes of fresh buy-out and early retirement offers.

The UAW has felt the pain in other ways too. The three carmakers and Delphi have forced it to accept a two-tier pay system in which wages for new workers have been cut in half to about $14 an hour with greatly diminished benefits.

A so-called Jobs Bank, under which idled workers are paid full wages and benefits just for showing up at the plant each day, is rapidly withering.

Nonetheless, the UAW is currently staging a last-ditch battle at five plants owned by American Axle, a big GM supplier, where 3,600 workers have been on strike since late February in protest against demands for similar concessions.