The "mission" of the Treasury under a Labour government would be to promote greater equality, Gordon Brown, the Shadow Chancellor, will declare in a speech today.
But he will insist the tax system is not the main means of achieving that aim, in order to persuade the voters that Labour is not a high-tax party.
In a speech at Manchester Business School, Mr Brown is expected to say he would write into the Treasury's "mission statement" a phrase from the new Clause IV of Labour's constitution, which would commit it to "create a fairer society with equal opportunities for all to work and prosper".
The move comes after three weeks of pressure on Labour to declare more detailed policy on taxing people on higher incomes, prompted by speculation by tax advisers and comments allegedly made by Tony Blair, the Labour leader, to journalists on the plane to New York that those on less than pounds 40,000 a year would not be hit.
Mr Brown's furious response forced one firm of accountants, KPMG, to cancel a series of tax avoidance seminars, while Mr Blair's aides denied he had mentioned any specific figures.
Mr Brown yesterday played down the role of the tax-and-benefit system in tackling the "massive inequalities" in Britain. "We have sought for too long to compensate people for the problems that have been created because of unfairness, instead of dealing with the root causes of unfairness," he said.
He said the solution was to enable people to acquire the skills to get them jobs, and reform the welfare state to encourage people to move off benefits into work. "We would like to get the level of taxation in the economy down."
He was backed by Kim Howells, a Labour trade spokesman, in a newspaper article yesterday. "Our values do not and have never included or implied higher taxes ... Indeed, many of our aims imply that success should be measured by how much we are able to reduce tax," Mr Howells wrote.
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