Speaking on the first day of the Association of Teachers and Lecturers conference in Cardiff, the union's general secretary, Peter Smith, said that by sticking to the spending pledge "like Araldite", Shadow Chancellor Gordon Brown would provoke cynicism among teachers and voters.
It was questionable whether the party would be able to tackle problems it had identified as needing attention, he said.
Mr Smith called on Labour to review spending on education within its first 100 days in power if it gained office on 1 May.
Increasing cash for schools should be its top education priority, since the sector had been squeezed "till the pips squeaked."
A recent Harris poll commissioned by ATL revealed 86 per cent of the public thought more money was top of the list if schools were to improve.
Mr Smith, who heads a moderate union of 150,000 teachers and lecturers, stepped back from condemning Mr Brown, but added: "If he really is saying that for two years he will apply the decision, with no moderation whatsoever, put into place by his predecessor Kenneth Clarke, then it will only be a matter of time before people ask, 'Time for a change? Where is the change in that?'"