Owners of properties worth between £2 million and £3m would pay an extra £3,000 a year under Labour's mansion tax plans, Ed Balls has confirmed.

But "asset-rich cash-poor" homeowners will be able to defer their bills until they sell or pass on their property, the Shadow Chancellor said.

Senior Labour figures have raised fears about the tax plans in recent weeks with Margaret Hodge, chairman of the powerful public accounts committee, warning it was "too crude to work properly" and former Labour cabinet minister Dame Tessa Jowell saying it could have an effect on families whose homes have gone up in value but whose incomes are low.

Mr Balls insisted that "modest properties" would not be caught up in the tax and underlined how the threshold for paying the charge would rise in line with house prices, rather than inflation, so the number of homes affected did not increase.

He said the tax would be progressive, with owners of properties worth more than £10m expected to pay substantially more than those at the lower end of the scale.

Homeowners with incomes on basic rate tax, under £42,000 a year, could defer the charge.

Mr Balls also reiterated that Labour are looking at targeting overseas owners of second homes in Britain in their plans to raise extra cash for the NHS.