The Conservatives will fight the next election on a pledge to cut at least one of Labour's tax increases, George Osborne has said.
The Shadow Chancellor has given a clear signal that a Conservative government would stop Labour's plans to increase national insurance contributions by more than £5 billion after the next election.
Alistair Darling, the Chancellor, last month set out plans to increase NI payments by workers and employees by 0.5 per cent from 2011.
The next election must be held by May 2010 at the latest.
Mr Osborne used a newspaper interview to say that his first priority in office would be to seek ways to meet the £5 billion cost of reversing the NI rises, either by cutting spending or increasing taxes elsewhere.
"My priority is to try to reverse the increase in National insurance because it is a tax that affects the vast majority of people in Britain," Mr Osborne said.
"It is a tax on jobs at a time of high unemployment. It is a tax on incomes at a time when people will be under severe strain."
The NI changes will cost workers around £2.5 billion, employers £2.65 billion and the self-employed another £250 million.
The Treasury points out that a decision to raise the starting threshold for employees will cut £1.6 billion from workers' contributions.
Labour's tax rises in future years will follow a short-term package of tax cuts including a temporary cut in VAT to 15 per cent.
Mr Brown is borrowing billions more to fund tax cuts, borrowing that will send Britain's stock of government debt above £1 trillion for the first time.
Ministers say borrowing more to support the slowing economy is the only responsible choice.
By contrast, the Conservatives have said they will only offer funded tax cuts, matching reductions in Government tax revenues with cuts in state spending.
Mr Osborne said there is a "moral argument" against running up larger Government debts. "Is it fair to burden future generations with the higher taxes and debts that you are not prepared to shoulder yourself?"
David Cameron, the Tory leader, underlined that message in the Sunday Telegraph yesterday, insisting that his party would stick to his message of "fiscal responsibility."
Opinion polls give Labour a narrow lead on economic matters and Labour strategists believe that the Tories are vulnerable to charges that they have not developed policies to meet the magnitude of Britain's economic troubles.
Angela Eagle, a Labour treasury minister accused Mr Osborne of making "empty promises."
"George Osborne is happy to try to grab headlines with vague talk about tax cuts but as usual he can't say anything about how he would pay for them," she said.
"The truth is these are just empty promises from the Tories. All that Osborne has confirmed today is that the Conservatives still intend to do nothing to give real help now to families.
Labour has also pledged higher taxes on high earners, including a new 45 per cent rate of income tax for people earning more than £150,000. Despite pressure from right-wingers, Mr Cameron and Mr Osborne have refused to say they would repeal that tax rate.