William Hague yesterday opened up a new front in the election tax war when he ruled out above inflation rises in national insurance charges.

The move came as the chancellor, Gordon Brown, came under continued pressure to rule out abolishing the ceiling on employee's contributions or raising it above indexation levels - as he has done in his last two budgets.

Conservatives are seeking to revive Neil Kinnock's 1992 "blunder" on NI contributions by raising the spectre of a threat to middle class incomes.

The Tories claim that Labour is planning to make up a £10 billion funding "black hole" by hitting families on over £29,900 a year by over £800. "If they cannot give the assurance we have given on NI contributions, people will have to conclude that they are planning to abolish the national insurance ceiling, hitting four million people." said shadow chancellor, Michael Portillo.

The party estimates that removing the cap on national insurance would be the equivalent of introducing a 50p top rate of tax on those earning above £34,000, a figure reached by combining the take for tax and NI.

Labour has accused Hague of opportunism as the measure does not feature in the Tory manifesto.

Gordon Brown, speaking on the BBC's Election Call refused to be drawn on the controversy saying he would not make a "whole series of additional promises" on tax.

However, pressure on the Chancellor to make a specific commitment on national insurance may be increased by figures from the Inland Revenue showing that employees paid £20 billion in NICs in 1997 but are expected to pay £25 billion this year.