A study by estate agent, Knight Frank, suggested that 775,500 properties would be subject to the tax during the next 25 years if property prices continue to rise at current levels. This compares with 55,000 homes worth more than £2m today.

It said the tax would generate around £1.3bn each year and that 86.4% of the affected properties were located in London and the South East.

Labour and the Liberal Democrats have been supportive of proposals to introduce a mansion tax and the former expected its introduction to generate £2bn for the government.

Knight Frank has now suggested this prediction was too optimistic and that all properties worth more than £1.25m would need to be included to raise such a sum.

Liam Bailey, head of research at Knight Frank, said: “Our calculations point to the real threat of the mansion tax threshold being lowered substantially in order to meet the revenue targets of the political parties.

“Even if the threshold is not lowered, it seems a fair assumption – given that it has remained at £2m since 2009 – that it would not be raised in line with future house price inflation thereby substantially increasing the number of properties affected by the tax.”