The dispute between Labour and the Conservatives over tax continued yesterday as Gordon Brown refused to rule out removing the cap on national insurance.

At a morning press conference, Brown and Tony Blair were visibly unsettled by questioning on the subject as Brown repeated his position of not giving any tax commitments beyond those in the Labour manifesto.

Shadow chancellor Michael Portillo seized on Brown's comments by saying the chancellor had been "rumbled".

"But increasing NICs for higher-rate taxpayers is in effect the same as introducing a 50p top rate of tax," Portillo said.

Last night, Brown was forced on to the defensive, giving a series of interviews to repair the perceived damage to his party's tax plans.

"What I am saying is that there will be no 50 per cent tax rate," Brown said before going on to describe Tory claims of a 50p tax rate as "smears that I utterly repudiate".

Last night William Hague, speaking at a rally in Plymouth sought to further exploit Labour's difficulty on the tax issue, claiming that 40,000 teachers, 20,000 police officers and more than 70,000 nurses would lose out under Labour.