Malcolm Turnbull has sought to sheet home a cut to penalty rates to Bill Shorten, as unions vowed a massive campaign to fight the decision.
Trade union officials were in Canberra on Tuesday to protest a decision by the Fair Work Commission to align Sunday penalty rates with Saturday's rates in the hospitality, retail, fast food and pharmacy sectors.
They claim the decision will affect 700,000 workers who risk losing up to $6000 a year.
"There has been a concerted attack on working people and now the icing on the cake is the penalty rates cut," ACTU president Ged Kearney told reporters at Parliament House.
"Families and communities will go backwards as a result of a pay cut the likes of which we haven't seen since the Great Depression."
Ms Kearney threatened a union campaign against the decision would be "widespread, long-lasting and very deep".
"All of our research shows that even Liberal voters are very concerned about this and it could be very damaging for those liberal members sitting right now in marginal seats," she said.
Mr Turnbull told parliament the Labor leader - a former Australian Workers' Union secretary - was the author of the penalty rates decision, having referred the issue to the Fair Work Commission when he was workplace minister.
"No individual in this parliament had more to do with setting up that review of modern awards and penalty rates than (Mr Shorten) ... he owns it, it's his process."
The prime minister said the government accepted the decision, which had been carefully considered by the commission.
However a parliamentary inquiry looking at the future labour force may add penalty rates to its ambit when it meets next week, its Liberal chairman Andrew Laming told parliament.
Mr Shorten said the government had a policy of giving more to companies when profits were surging and cutting pay when wage growth was flat.
Labor seized on a comment by Liberal MP Ann Sudmalis who said cutting penalty rates were "a gift for our young people to get a foot in the door of employment".
"This pay cut isn't a gift, it's an insult to 700,000 working Australians," the opposition leader said.
Mr Turnbull told MPs Ms Sudmalis was a school teacher before she entered parliament.
"She knows the situation of young people in the electorate of Gilmore very well, and it's very heartfelt." he said.
Labor workplace spokesman Brendan O'Connor acknowledged it was an "extraordinary step" for the party to question the long-standing independent umpire.
"But it is an extraordinary decision," he told Sky News.
