David Miliband, the Labour leadership frontrunner, must ditch his attachment to Blairite policies on privatisation and globalisation if he is to avoid splitting the party, the leader of Britain's biggest public sector union insists today.

Dave Prentis, the general secretary of Unison, said that Labour was at a watershed moment in its relations with the unions and accused the elder Miliband of having been part of a New Labour elite which caused untold "trauma" to public sector workers and sought to "beat up" unions. The comments, in an interview with the Observer before next week's trades union congress, are part of an attempt by the unions to reassert their influence, after years of being sidelined, as the Labour party prepares to choose a new leader.

They also suggest that after the new leader is announced on 25 September, Labour will be plunged into a heated argument at its party conference on its future direction. Prentis, whose union is backing the more left-leaning Ed Miliband in the election, said that Ed Miliband reflected the values of the 1.4 million public sector Unison members "far better than the other candidates".