She has since sought to backtrack and state that she was speaking generally about those in society who could afford to pay more.
But the blunder – the latest to hit Ed Miliband’s party, amid low polling ratings – has been seized on by the Conservatives who have incorporated it into their latest election poster.
During a phone-in on LBC radio in London, Ms Harman was told by a caller named Henry: “I really do feel the middle class contribute the most and take out the least. I will be honest I am a staunch Conservative, hand on heart but if Labour came up with one policy that would genuinely help me out I would vote for them.”
Ms Harman responded that public services such as health and education were available to both middle class and lower income families. She added: “Yes I think people on middle incomes should contribute more through their taxes. But actually they need those public services like the transport system.”
The comment was seized upon by David Cameron in Prime Minister’s Questions.
He demanded that Labour comes clean over what new taxes they will “hit the squeezed middle” with.
During exchanges with Mr Miliband on the cost of living, Mr Cameron said: “One of the things that wasn’t noticed but happened [Monday night]: the deputy leader of the Labour Party on the radio said this – and I want to quote it very precisely – ‘I think people on middle incomes should contribute more through their taxes’. That is what she said.”
He continued: “That is their policy. The squeezed middle will be squeezed more. Now you need to tell us which people are going to pay which taxes because on this side of the House, we have cut council tax, we have cut petrol duty, we have cut the jobs tax, we have increased the married couples allowance.”
Later, in a sign of the beginnings of a bitter 10-month election campaign, the Tories previewed a new poster of Ms Harman warning voters about a Labour tax rise.
Tory party chairman Grant Shapps said: “Ed Miliband defines middle income as ‘people either side’ of £26k a year. Now Labour say they should pay more tax.
“It’s clear that Ed Miliband would crush middle Britain. He wants a new tax on your family home, higher fuel duty, a rise in corporation tax, and an extra tax on your pension.”
But Labour hit back, calling the Prime Minister “dishonest”.
They claimed Ms Harman was referring to the contribution made by middle earners in the current tax system, not any future plans.
A visibly furious Ed Miliband was forced to field questions on the issue at a Westminster photo opportunity for Labour’s battle bus for the Scottish referendum campaign.
He described Mr Cameron as “dishonest” and added: “Harriet Harman was very clear that we are in favour of cutting taxes for low and middle income earners with things like the 10p rate.”
Ms Harman wrote to Mr Cameron yesterday accusing him of telling “a lie”.
She added: “As you surely know, since your own party circulated a transcript later, I had made a straightforward defence of our system of progressive taxation – the idea that people on higher incomes should – and do – pay more in tax overall than people on lower incomes.”
Speaking to The Scotsman at the campaign event, she insisted the remark on LBC radio was “neither a gaffe nor letting the cat out of the bag”.
She added: “I am disappointed in the Prime Minister.
“This is a great office of state and the voters deserve better than that.”