The Liberal Democrats have claimed the Conservatives would be forced to raise VAT to pay for promised tax breaks if they won the General Election.
The party's new campaign poster warns that the increase to 20.5% would cost an average family £389 a year.
In a speech in Glasgow, Lib Dem leader Nick Clegg will claim that only his party would cut taxes for people on an ordinary income.
The Lib Dems claim that Conservative proposals to recognise marriage in the tax system, reduce the impact of the planned National Insurance hike, freeze council tax and raise the inheritance tax threshold will cost £13.5 billion.
Mr Clegg will say David Cameron has offered only "vague and unconvincing" statements about how he will pay for the promises he has made.
He will say: "The Conservatives have made a series of tax promises in the last few years. Tax breaks for millionaires with a £2 million exemption from inheritance tax. Tax breaks for married couples. A freeze in council tax.
"Reducing the impact of Labour's proposed rises in National Insurance. And another National Insurance tax break for small businesses. Paying for all these tax promises would cost £13.5 billion. And the Conservatives have identified a paltry £100 million of alternative tax revenue to pay for it. Together with a series of vague and unconvincing statements about increased efficiency which David Cameron himself says are a trick.
"It is time for the Conservatives to come clean about how they will pay for these tax cuts ... without decimating schools, hospitals and police forces across the country."
He will say the only way to find the money would be a tax rise, claiming the Tories had shown in the past they "love to raise" VAT.
"It's the Tory VAT bombshell. Under the Conservatives, the richest 3,000 families in the land will be £233,000 better off because of inheritance tax cuts. And everyone else will be stung for nearly £400 more in VAT."