The mansion tax has taken a step nearer reality, with business secretary Vince Cable spelling out the proposition.

He says the annual tax will be levied at 1% on properties worth £2m-plus, with the levy charged on the excess over £2m.

However, writing in the Mail on Sunday, Cable makes it clear that he hopes to extend the tax.

He says: “I have not given up hope of persuading my Cabinet colleagues to go further. We may yet get agreement to widen the mansion tax.”

Cable says the tax will tackle the “chronic unfairness in property taxation” and insists that his views are widely shared. The Labour party also proposes a mansion tax while Cable says that a YouGov poll shows that “considerably” more Tory voters support the idea than oppose it.

Cable says that most of the mansion tax would come from London and does concede that there are some people – but only a small number, he claims – who have property wealth but little income.

This problem, he says, could be dealt with by rolling up tax obligations against a future sale, as already happens with large care home bills.

Cable does not spell out in his article how properties would be valued for the purposes of mansion tax, nor exactly when it would be introduced.

Labour believes there are currently some 70,000 properties worth £2m-plus, and that a mansion tax could bring in £2bn.

Meanwhile, the current government’s determination to squeeze as much out of residential property as possible has been underlined by a forecast that the take from Stamp Duty will almost double by the year 2018/2019.

The forecast is from the Office for Budget Responsibility in its analysis of last week’s Budget.

It says Stamp Duty receipts are set to increase by 90% over the next four years from £9.5bn this year to £18.1bn in 2018/19.