Conservatives may not reveal details of how they plan to cut £12 billion from benefits before voters go to the polls on May 7, Work and Pensions Secretary Iain Duncan Smith has said.

Mr Duncan Smith said that the planned changes will have a "life-changing, dramatic" impact on claimants, arguing that they will improve lives by allowing people to get off welfare and back to work.

His comments came as the independent Institute for Fiscal Studies (IFS) warned that the planned cuts will be "really tough" to achieve, involving "pretty dramatic" reductions in areas such as housing and disability benefits over the next three years.

IFS director Paul Johnson said that cuts under Labour would be "quite a lot less" but could involve "big cuts" in the first couple of years after the election and would leave the country with a deficit of up to £30 billion - while Tories would eliminate it altogether.

Mr Duncan Smith dismissed a leaked document which appeared to suggest his Department for Work and Pensions was considering changes to industrial injuries compensation, child benefit, the carer's allowance and disability benefits. It was one of a number drawn up by civil servants.

The Work and Pensions Secretary told BBC1's Andrew Marr Show that the Conservatives had already announced around a quarter of the £12 billion cuts they have pencilled in for working-age welfare benefits, by extending a freeze for a further two years and reducing from £26,000 to £23,000 the cap on household claims.

But he said the party may not think it "relevant" to explain where the rest of the cuts will fall before the election.