George Osborne "must" make the wealthiest pay more to help fund tax cuts for the less well off in next week's budget, Nick Clegg will tell his party's faithful on Sunday.
In his keynote speech to Liberal Democrats at the party's spring conference in Gateshead, the Deputy Prime
Minister will publicly call on the Chancellor to include "concrete" measures supporting lower earners.
The move, which aides described as "raising the stakes", comes ahead of a crunch Coalition budget meeting on Monday.
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Mr Clegg will say the Lib Dems have secured major concessions on tax for poorer households but admit there is "still more to be done".
"That is why the Budget in ten days time must offer concrete help to hard-pressed, hard-working families," he will add.
"A big increase in the income tax threshold, further and faster towards £10,000. Help we promised, help we are delivering, in Government, today.
"I want the budget to show how we are anchoring this Government in the centre ground. Credible - but fair.
"The last Labour budgets led our nation to the economic precipice - fantasy budgets issued by a party in denial, out of ideas and abdicating responsibility.
"This month's budget will show the determination of both parties in government, to repair the public finances, keep our economy safe and help working families.
"This year's Coalition Budget must cut the taxes of ordinary working families by asking more of wealthiest.
"The wealthy paying more so that ordinary workers pay less, a budget for fairness."
The Lib Dems have floated a number of ideas for hitting the wealthiest, including proposals this weekend for a so-called "tycoon tax".
Party sources insisted it was "in the budget mix" but conceded much of the detail of how that would work in practice was yet to be thrashed out.
It could include looking at the overall income of the wealthiest, including payouts from things like dividends and trust funds, then setting a minimum level of tax payable on the total.
Proposals for a "mansion tax" are unlikely to get past the Conservatives, who are strongly opposed to the plans.
But the March 21 budget must tackle tax avoidance, Mr Clegg will tell activists.
"Too often, rather than paying their dues to the nation the wealthy pay their accountants to get them out of it.
"Avoiding tax, minimising the amount they have to contribute - that's the name of their game, boasting about the latest wheeze for moving an asset here, a property there, a loophole everywhere, all to make the tax bill lower.
"Let me tell you, few things make me angrier as ordinary families struggle to make ends meet, as young people struggle to get on the housing ladder, as the unemployed struggle to find work, the sight of the wealthiest scheming to keep their tax bill down to the bare minimum is frankly disgraceful, multimillionaires avoiding tax by moving their money around.
"So, we will call time on the tycoon tax dodgers and make sure everyone pays a fair level of tax."
Mr Clegg's speech will not contain major new policy announcements but is instead expected to focus on "positioning" the party as well as setting out its achievements in coalition.
He will make a robust defence of controversial reforms to welfare to show the Government is completely unified on the issue, even if it may be for different reasons.
The Lib Dem leader will say benefit claimants "owe it to the nation" to "strain every sinew to find a job".
"That's why I am such a strong supporter of the basic idea driving the Coalition's welfare reforms," he will add.