Research from law firm Clarke Willmott LLP shows that around 485,000 of London’s properties are owned and occupied by pensioners.

With average house prices in the capital surpassing £362,000, according to Nationwide, this suggests more than half of London’s owner-occupier pensioners will be over the £325,000 inheritance tax threshold based on the value of their property alone.

Andy Kirby of Clarke Willmott said: “Most pensioners will certainly not think of themselves as rich and may not have considered the significant inheritance tax their beneficiaries will face in future.

“With rents as well as house prices very high, there is a growing trend for children to live at home well into adulthood. Parents of these children may believe they are leaving a roof over their heads when they pass on, but the reality is that they may be forced to sell the family home simply to release the funds needed to pay inheritance tax.”

The Clarke Willmott figures follow a report published last month by accountants UHY Hacker Young which revealed that Britain’s families pay the second highest death duties in the world.

Only Ireland has a higher rate of inheritance tax than Britain.

The inheritance tax threshold has been frozen at £325,000 for individuals and £650,000 for couples until at least 2017/18.

But David Cameron has pledged to raise it to £1m at the next election.