ED Balls has been accused of "letting the cat out of the bag" on future tax rises under a Labour government after leaving the door open to drawing hundreds of thousands more middle-income earners into the 40p income tax band.

The Shadow Chancellor, in an interview, repeatedly refused to rule out trying to balance the nation's books and reduce the £90bn budget deficit by lowering the amount people have to earn before they pay the higher rate.

During the past 20 years, the number of workers who pay the 40p rate has already risen substantially from two million to five million, bringing in billions of pounds to the Treasury.

The Tories have pledged that if they won power on May 7, they would raise the threshold at which people paid the 40p rate from the current £42,000 to £50,000.

But Mr Balls, in an interview with ITV West Country, was asked whether or not he was "leaving the door open" to changing the 40p threshold and if people could be "paying more tax" under Labour.

He replied: "What I would like to do is find ways in which I could have fewer people in the 40 per cent tax bracket; of course, I would. But I have to be honest with people. The deficit is going to be £90bn."

The Labour frontbencher went on: "I have got to find a way to get the deficit down in a careful and staged and balanced way. And I've set out the tax rises I'll need on the very highest incomes. And I've said that for working people I'm not going to raise the VAT or National Insurance. I'm not going to be raising the higher rate of income tax."

Pressed on potential changes to the income tax bands, Mr Balls insisted he could not go through every detail, saying: "I can make a promise not to increase the basic rate of income tax or the higher rate of income tax. The threshold is also an important detail and that's complex."

But George Osborne seized on his opposite number's remarks. "Ed Balls has let the cat out of the bag and confirmed a tax assault on middle earners. He has confirmed that he and Ed Miliband will put up taxes on Middle Britain; it is part of Labour's £3000 tax rise for working families.

"By contrast, the Conservative plan is to increase to £50,000 the threshold at which people pay the higher rate of tax. So there is a clear choice," declared the Chancellor.

In response, Labour HQ stressed how under the Conservatives around two million people had been dragged into the 40p tax band and that the Opposition had backed the increases in the 40p threshold in the recent Budget.

"Tory promises to raise the 40p threshold further are completely unfunded and will cost billions of pounds. Together with their extreme spending plans, these unfunded commitments will mean the Tories end up raising VAT again and cutting the NHS," said a spokesman.

He stressed that when Labour made tax promises, such as introducing a lower 10p starting rate of tax to help millions of middle and low earners, it had set out where the money was coming from.

"Working people are paying more under this government, which is why the only tax changes we are proposing are on the very richest in society like reversing the Tory tax cut for millionaires," added the spokesman.

In the final days of the UK Parliament, David Cameron pledged to rule out raising VAT if the Tories won power while Mr Balls later ruled out Labour increasing National Insurance Contributions if it won the election.