Unison boss Dave Prentis took aim at the heavily pregnant Duchess of Cambridge in a speech attacking government cuts to welfare.
Labour leader Ed Miliband was urged to distance himself from the ‘outrageous’ attack just weeks before she is due to give birth.
Outrageous: Dave Prentis, general secretary of the Unison union, likened the Duchess of Cambridge to a benefit scrounger
He condemned the coalition’s ‘scorched earth policy’ on welfare payments, adding: ‘If we stopped the poll tax, we can stop the bedroom tax.’
Mr Prentis went on: ‘In the tabloid press, so much is written about benefits abuses - young women having babies to get state hand-outs.
‘But conference, that’s enough on Kate Middleton.’
The slur comes just days after the Duchess made her last solo public appearance prior to the birth of her first child next month.
She launched the Royal Princess cruise ship with a £1,250 Nebuchednezzar of Moet et Chandon champagne in Southampton on Thursday.
And on Sunday she attended the Trooping of the Colour where brother-in-law Prince Harry made her laugh with a tickling gesture.
In 2010 Unison backed Mr Miliband's leadership campaign against his brother David.
Since Mr Miliband became leader, Unison has donated almost £4 million to Labour Party coffers.
Naming: In her last solo public engagement last
week, the Duchess of Cambridge watched as a bottle of champagne broke
against a cruise ship during a naming ceremony in Southampton
Speculation: The royal baby is due in July, but
with weeks to go it has not been announced whether the couple are
expecting a boy or a girl
Tory MP Henry Smith said: ‘This is an outrageous personal attack on the Duchess of Cambridge.
‘Ed Miliband should distance himself from these spiteful remarks.
'If he does not, it will be clear that he is a weak leader who stands for the union barons' interests, not the interests of hardworking people.’
Labour last night declined to comment on Mr Prentis's remarks.
He said: ‘For too long we're built the careers of Labour politicians, only to be let down when we needed them most.
‘I don't want to hear Labour apologising for past mistakes, I want to see a clear agenda from Labour for the future.
‘We must not support a Labour Government that does not: put an end to privatisation and market madness or restore our NHS- invest in our public services, restore the facility time taken away from our activists, restore workers rights and remove the shackles on trade unions.’
The Duchess of Cambridge has faced repeated criticism since marrying Prince William two years ago.
Author Hilary Mantel’s described her as a ‘shop-window mannequin’ with a ‘plastic smile’.
And newspaper columnist Joan Smith dismissed the Duchess as ‘unambitious and bland’ and Britain’s ‘Queen Wag.’
Mr Prentis's jibe was dropped from an official transcript of his speech posted on the union's website.
Mr Prentis also hit out at the cost of Baroness Thatcher's funeral, saying the Tories had spent £10 million to 'assuage their guilt' at having toppled her and describing it as a 'waste of public money'.
In fact the cost to the taxpayer of the funeral was £3.6 million.
And he described UK Independence Party leader Nigel Farage as a 'conman extraordinaire', likening him to Enoch Powell.
Mr Prentis accused the Government of launching 'the most vicious assault on the welfare states' which had led to the 'cruellest attacks yet on the weak and vulnerable'.
