Income and other benefits can be paid out to beneficiaries, who may or may not include the trustees, and who will be taxed on them as normal.

Peter Legg, a chartered tax adviser and founder of IHT Planning Matters, said: "Here it would appear that shares in the businesses are owned by family members as trustees, not as individuals." This puts the assets at arms' length and effectively eludes death duties.

Mr Clark said: "The Duke and other family members are likely to be part of a 'beneficiary class'. That means they could receive income from the assets.

"But they do not have entitlements to either the income or the assets themselves."

Mr Clark speculated that, depending on how the trust was established, all six trustees might need to agree significant transactions.

If the Grosvenor family can use this strategy to avoid death duty, can anybody?

In theory, yes - though costs could be prohibitive.

"You need to have a seriously large estate to make such a structure feasible," said Mr Legg. "The costs of setting up a trust and the ongoing reporting costs can be onerous."

More significant though are recent tax changes, which have made these types of trust less attractive to those who want to establish them afresh.