The mansion tax, which is supported by Labour and the Lib Dems, would be levied at 1% a year on all properties valued at more than £2m.

There are 55,000 home in the UK which are estimated to fall into this bracket, Labour claims that the tax could raise £2bn a year for the Treasury.

The Tories are dead set against introducing such a tax, which they claim would be costly and time-consuming to administer, requiring houses to be revalued to see whether they were liable.

They believe such a levy would penalise pensioners who have lived in a property for a long time, saved up and improved it.