When queens grow old, a blind eye is turned to their personal finances. Such was the case with the Queen Mother, whose extravagant expenditure on racing and entertaining in her 90s was indulged with barely a murmur. Similarly no one seems the least perturbed about how our nonagenarian Queen is spending the extra millions that have recently swelled her privy purse.

Without fanfare it was disclosed on Tuesday that Her Majesty had hit the jackpot, receiving a record £19.2m from the Duchy of Lancaster in 2016-17, up 7.9% on last year’s profits.

Revenue from the duchy – the ancestral estate that has for the last eight centuries provided a private income to the monarch – has trebled since the millennium. If the Queen could get by on £5.8m in 2000, why does she need almost £20m today?

The mushrooming income of Queen Victoria in her dotage famously stirred critics to cry: “What does she do with it?” – and the same question might be asked today of her great-great-granddaughter.

To be fair, the Queen’s outgoings are high, what with the maintenance fees of running the vast country estates of Balmoral and Sandringham and paying for a string of pedigree racehorses. But these costs are relatively constant.

What has grown is her family. Almost certainly the Queen’s Lancaster hotpot is used to feed her hungry brood of children – particularly Princes Andrew and Edward, who maintain lavish lifestyles without any discernible earned income.

No doubt Princess Anne also receives a generous helping from her mother in return for fulfilling official engagements – as do probably those other active royals Princess Alexandra and the Dukes of Gloucester and Kent.

This family cross-subsidisation dates to 1992 when, after a shake-up of the royal finances, it was agreed that civil list annuities would be limited to Prince Philip and the Queen Mother and that the Queen would effectively self-finance the royal duties of her other close relatives. At the time the bill was about £1.5m; today, with the deaths of Princess Margaret and the Queen Mother more than offset by the arrival of new royals, it is sure to be far higher.

Given that the Queen has a good chance of living as long as her centenarian mother, there are now grandchildren and great-grandchildren to provide for and, given that her mother and father are known to have set up trusts for their close family members, it’s likely that Her Majesty has done something similar – and at some cost.

Such family generosity would be impossible without the Duchy of Lancaster, the long-standing wellspring of the Windsor wealth. When the Queen acceded to the throne in 1952, its revenue was a modest £100,000 a year. This grew steadily until the millennium when, with commercial rents soaring, profits dramatically doubled in a decade, reaching £13.2m in 2010. Today, total duchy assets are worth more than half a billion pounds. For decades parliament has struggled to get to grips with the estate’s finances. The income it provides the monarch may be classified as private, but is the capital – which cannot be accessed by the monarch due to its trust-like status – public? And, if so, shouldn’t it be subject to parliamentary scrutiny like other public assets? Insisting that “it’s a private estate”, duchy officials have long been loath to open up the books to parliament’s financial watchdog, the National Audit Office.

But now that Prince Charles has disclosed in his annual review how much of his Duchy of Cornwall profits go to his children, any audit of the Duchy of Lancaster should expect similar information from the Queen.

If money is being given by the head of state to her children to perform public duties, then surely the public has a right to know how much. By the same token, if some of the cash is being used to finance their private lives, then this too should be made transparent.

Full disclosure could also address the key issue of assessing how the duchy profits correspond to the actual needs of the Queen’s household. This was one of the recommendations of the Public Accounts Committee report which scrutinised duchy revenues in 2005. In a damning phrase, it argued that the income was “an accident of history” stemming from arrangements established in the 14th century which might not be appropriate today.

Since the report’s publication over a decade ago, the duchy’s profits have more than doubled. Dare one ask if the Queen receives far more than she requires? And if she’s splashing the cash on the kids, shouldn’t the tap now be turned off so that they can earn their own corn?