Critics of inheritance tax– dubbed the “death tax” – suggested the whole system should be ended amid evidence the prime minister’s family had attempted to minimise payments.

One Tory MP said: “I’m for scrapping the death tax. If we want people to save money to pass on to their families, why do we discourage them by taxing them again? It doesn’t raise a huge amount of money.”

Mr Johnson said that polls suggested the tax was “incredibly unpopular” and suggested a review into the scheme was long overdue. “I certainly think that there’s a case for reviewing how the whole thing works,” he told The Telegraph.

“I’m not aware of their having been a government-sponsored review since the old capital transfer tax was turned into inheritance tax some time in the early 1980s.There have been some changes, but there hasn’t been any substantive reform to it for thirty years.”

Mr Johnson added: “One of the reasons it seems unfair is precisely because only the really pretty wealthy can pass on significant sums during life that don’t attract inheritance tax.

“For most people their wealth is in their pension and in their house. Most people can’t pass that on in their life. Only if you’ve got a few hundred thousand pounds sitting around in bank accounts can you pass it on. That it just not available to most people.”

Ben Riley-Smith, Political Correspondent