The Tories plan to open a new front in the general election campaign by detailing how they would save millions of workers from Labour's planned National Insurance hike.

In a move that will create a clear dividing line between the two parties, shadow chancellor George Osborne will pledge to reverse part of the increase due to take effect in April 2011.

The Tories' plans would mean that seven out of 10 workers will not be affected by the 1 per cent increase in contributions for those earning more than £20,000. Top earners will still be hit.

It is understood that the move, which will cost several billion pounds, will be funded from spending cuts rather than tax rises elsewhere. This will allow the Tories to fight the election on a platform of lower taxes, although Labour is bound to try to discredit their figures.

The increase in NI, for employers as well as employees, was pre-announced by Chancellor Alistair Darling in 2008 and 2009 as he struggled to balance the books as borrowing soared.

Mr Osborne attacked it as a tax on jobs and identified its reversal as his top priority if he becomes chancellor after the expected May 6 election.

Polls at the weekend continued to point to a hung parliament, despite voters' apparent dissatisfaction with last week's Budget.