Nick Clegg, the Liberal Democrat leader, will tonight reaffirm his demand for tax cuts worth £18bn for low and middle income earners.
With Labour and the Conservatives proposing rival tax-cutting strategies in the hope of reviving the economy, Clegg will criticise both parties for the "small-mindedness" of their plans.
And he will claim that Lib Dem policies could "fundamentally rebalance Britain's unfair tax system", as well as delivering a boost to the economy.
"We hear talk of tax cuts emerging from Downing Street, but they are likely to be small, and short term," Clegg will say in a speech to the Royal Commonwealth Society.
"Funded through borrowing, the money will have to be paid back later. So it's meagre tax cuts today, giant tax rises tomorrow from Brown.
"Meanwhile, the Conservatives want a piffling incentive for businesses to take on new workers that won't put a penny in the pocket of a single family in Britain. Neither package comes close to what's needed.
"No wonder people are cynical about politics. All they get from the government and the official opposition is timidity and tinkering."
The Lib Dems are already committed to cutting the basic rate of income tax by 4p in the pound. Clegg will say this would result in a worker on £30,000 receiving almost £1,000.
The party would meet the £18bn cost of this by cutting top-rate tax relief on pension contributions; taxing capital gains at the same rate as income; imposing green taxes; and tackling corporate tax avoidance.
Clegg will say that these are real tax cuts, "big, permanent and fair", that would benefit those who need help.
He will also call for the government to clamp down on tax havens.
"There's a particular UK responsibility here, since half the world's significant tax havens are under British sovereignty," he will say.
"I know tighter regulation of offshore companies and banks will be painful for some small Commonwealth states and British dependencies. But it has to happen.
"Up to £40bn is lost every year in the UK alone through tax avoidance, much of it through offshore havens. That's money that should be recycled into tax cuts for ordinary families."
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