Vince Cable, the Liberal Democrats’ Treasury spokesman, unveiled plans yesterday for a property tax rise on millionaires, opening clear water with the Conservatives by up-ending their scheme to cut inheritance tax.

But the proposed annual levy on homes over £1m, designed to show the Lib Dems were 'committed to fairness', was quickly deemed unworkable.

Cable said at the party’s conference in Bournemouth that collecting half a penny in the pound on properties worth more than £1m was a “simple and easy” way to raise enough revenue to lift 300,000 low-paid workers and pensioners out of tax. About 250,000 people would pay an average of £4,000 a year, according to Lib Dem estimates, raising a total of £1.1bn.

'We have seen the superrich pouring their money not into job-creating businesses but into acquiring mansions,' Cable said.

Financial Times