It was Roy Jenkins who as usual came up with the killer question. How senior does a royal have to be — he asked one palace official — to preclude him or her from holding a regular job? The issue was raised by the former Labour Chancellor of the Exchequer during a hearing of a parliamentary review of the royal family's Civil List funding back in 1971 but it remains equally relevant today as Prince William will tell you.

The man born to be king made waves on a visit to a youth centre on the Cornish coast on Thursday (1 September) when he admitted that "it took me an awfully long time to work out what I wanted to be". One seasoned royal watcher likened his plight to that of Hamlet, another prince with existential doubts and a problematical step-parent.

William's predicament raises the broader question of what sort of work royals can be expected to do. If the second-in-line to the throne is likely to wait many a decade before he gains the top job (remember his father has been Prince of Wales for almost half a century) then wouldn't it be in everyone's interests if he found some purposeful employment outside his official duties? But just how close to the real world can that work be?

Traditionally monarchs have dealt with the dilemma by marching their children straight into the armed forces. It's a secure environment cut off from politics, social scandal and — crucially — media intrusion. Alas, as William found in September 2013 when they privatised his previous job as a RAF search and rescue helicopter pilot, the forces are shrinking fast and there are now many fewer posts suitable for royals.

If the army is hors de combat, then why not the arts as an alternative field of employment? But here there's a world of difference between being a hands-on administrator and a glad-handing patron — as the career trajectory of two royals testifies.

Lord Harewood — the Queen's cousin — loved opera and successfully ran two national opera companies (casting manager, Royal Opera House 1953-60; managing director, Sadler's Wells 1972-85). Princess Margaret — the Queen's sister — harboured an equal passion for ballet but rather than having a professional role, hers was limited to one of patronage (president Royal Ballet 1957-2002; patron, English National Ballet 1976-89).

On their deaths, Harewood, aged 88, was universally lauded for his substantive contribution to the arts. It was "unusual for a member of the royal family to gain praise on account of what he did rather than who he was", one obituarist noted. Whereas Margaret , aged 71, was widely lambasted for not putting her talents to better use. According to Gore Vidal, "she was far too intelligent for her station in life".

Could Prince Harry be following in her ballet footsteps? Like his great aunt he loves to party. And like his big brother he left the armed forces early — no doubt feeling uncomfortable in a MoD desk job after seeing action in Afghanistan. But the experience of other junior siblings finding gainful employment after the army is far from encouraging.

Ex-marine cadet Prince Edward set up his own television company Ardent Productions in 1993, allowing him to cash in on his royal connections and make documentaries about Windsor Castle as well as several of his fellow Windsors. But despite an investment of £2.2m, the business was never genuinely profitable, finally going into voluntary liquidation in 2009 with assets of just £1,440.27.

If private enterprise is too risky, couldn't they try one of the professions? Many forget that residing in the quieter wing of the House of Windsor is a trained architect — Richard, Duke of Gloucester, RIBA. After qualifying from Cambridge University he went into practice as a partner in a north London architectural firm. Sadly, the sudden deaths, within a couple of years of his elder brother and father forced him to swap a promising career at the drafting table for the responsibilities that go with the ducal title. Later his mother, Princess Alice, observed wryly: "As parents we could never quite make out whether our children were supposed to lead royal lives at the command of the Queen or were free to follow professional careers of their choice."

The careers of Richard's deceased relatives offer a pointer to two other areas of productive work. In the late sixties his brother William passed the exacting Foreign Office entrance exams and was posted as Third Secretary to Nigeria and then to Japan. The work broadened his horizons in every sense.

There's no obvious reason why another Prince William could not join the FCO – particularly since he's already gained much experience as a de facto ambassador for the UK on his recent overseas tours of North America, Australia and India.

Were ambassador deemed too lowly a position for the future monarch, William might prefer to upgrade to governor-general. This was the route taken by the previous Duke of Gloucester, Prince Henry, when at the height of a fulfilling career in the army in December 1944 he agreed to go to Australia for two years to serve as governor-general.

Anti-monarchist sentiment might rule out a posting Down Under for William but there is no reason why he could not emulate his great-great uncle, the Duke of Windsor, and become governor-general of Bermuda — a haven for his young family far from the metropolitan media spotlight yet close to the mainland for official tours to the US and Canada as well as visits to Caribbean Commonwealth nations.

Roy Jenkins was clearly on the right track in drawing a red line between senior and junior royals. William needs to find some useful employment in his protracted wait to be king but as the second in line he can hardly be expected to go into private business like Prince Edward to support his family.

But why the need for junior royals at all? When the future strategy of "The Firm" has been discussed, even the Queen has been known to sigh, "there are just too many of us". Wouldn't it be better for everyone concerned if the active royal family were limited to, say, half a dozen royals who when not carrying out official engagements could serve in the Foreign Office, Ministry of Defence or armed forces? The rest of the family could then be free to pursue their career of choice — be it in the arts, the professions or business. Prince Edward might even be prised out of retirement to make a documentary about the fish-out-of water experience.

David McClure is the author of "Royal Legacy" published by Thistle Books, 2015