Much like his late grandmother, the future King knows the value of relying on close relatives for support

Almost all of the 8,000 guests at this week’s Buckingham Palace garden party received their invitations through the post, on thick, cream-coloured card with embossed letters fit for royalty.

Five of them got a WhatsApp message – from their cousin, the Prince of Wales, who thought it might be nice to include them.

Those five – Peter Phillips, Zara and Mike Tindall, Princess Beatrice and Princess Eugenie – duly rallied, gathering for a catch-up with Prince William inside the palace before stepping out under umbrellas to bring small talk and laughter to the rain-soaked crowds.

Their arrival lifted what could have been a rather different occasion for the Prince, who was hosting the garden party on behalf of his father and without his wife.

Princesses Beatrice and Eugenie offered a show of support for their cousin Credit: Yui Mok/Reuters

Standing for God Save the King alone, with the working Duke and Duchess of Gloucester standing a little behind him, the Prince could have cut a rather lonely figure without the Princess, who is at home recovering from illness.

Instead, afterwards, with Mike Tindall, the rugby-playing cousin-in-law who calls him “One Pint Willy”, to hand, William led the younger royals in a cheery, photo-friendly tour de force through the gardens.

The “slimmed-down monarchy” this was not. Here, subtly, was a glimpse of the instincts of the future king – more visibly in line with his late grandmother than ever.

Just as Elizabeth II relied on supportive cousins of her own generation, as well as loyal children and grandchildren, to join her at public events, so the Prince sought out the family he grew up with. The reaction from the public was encouraging.

Elizabeth II and her cousin Lady Mary Colman in 2013 Credit: Max Mumby/Getty

“The cousins are all very close and always have been,” said one source. “I think people liked seeing them being there together and supporting each other.”

Princess Eugenie wrote on Instagram afterwards: “Come rain or shine, I was delighted to support my family to meet some special individuals at the Buckingham Palace garden party who have gone above and beyond to support their local communities and the country.”

Palace sources have been swift to downplay suggestions that any of the younger generation will be making an imminent return to official working duties. The “no half in, half out” rule still applies.

Some, however, have detected an appetite for more casual but visible public appearances from the late Queen’s “non-working” grandchildren that until recently were thought to be a thing of the past.

The Prince of Wales, The Telegraph understands, would be keen to extend similar invitations again, in the spirit of including the wider family, as his grandmother once did.

The King, too, has seemed visibly buoyed by the company of his nieces and nephews, rarely looking happier than he did clasping Zara Tindall in a hug at the Royal Windsor Horse Show this month on his way back from cancer treatment.

He must now decide what to do for this year’s Trooping the Colour, in which the state of the Royal family will be captured in that all-important “balcony moment” displaying who is in and who is out.

The mood is far from the scene of 2012, when the late Queen’s Diamond Jubilee was marked by a photograph of that much-touted slimmed-down Royal family in action: Elizabeth II, the then Prince of Wales and Duchess of Cornwall, the Duke and Duchess of Cambridge and Prince Harry.

Rumbling for years, the “slimmed down” policy has long been attributed to the King, who was conscious of the cost of what seemed at the time to be an ever-expanding Royal family.

As the transition to his own reign began, the future was very clearly intended to be the direct line of succession: Prince William and his nuclear family, with self-proclaimed “spare” Prince Harry and family included. The departure of the Sussexes, as is well known, changed everything.

Since the King and Princess of Wales have taken time away from public duties for cancer treatment and preventative chemotherapy, so concerns over the “slimmed down monarchy” theory have continued.

Insiders reject the idea that it has ever been an official policy, or that there is any plan to reverse it. Often, one of them says, the Royal family acts as any other family, inviting relatives along without any grand theory behind it.

“They’re very willing to step up and do more at this current time, to help,” said one royal source, of the younger generation. “They’re very fond of their cousin and their uncle, and they want to do everything they can to support them. And they believe in the institution they grew up in.”

Peter Phillips, and Zara and Mike Tindall, are doing so on an ad hoc basis, when asked, and are sure to continue the tradition of Royal Ascot in a few weeks’ time. Princess Beatrice and Princess Eugenie, who have full-time jobs, are regularly rumoured to be on the cusp of being elevated back into the royal centre-stage.

Mike Tindall seemed content joining in to entertain guests at the party Credit: Yui Mok/PA

The charity work of Princess Beatrice, in particular, is becoming more prominent. This week she conducted her first ever live interview – from New York for an Outward Bound Trust event, broadcast on ITV – and went to a primary school to read with her patronage Oscar’s Book Club.

“I don’t think it means there’s a plan for them to be full-time working members of the Royal family and I’m not sure they’d want that?” said a source of the York sisters. “They have careers and families and they’re very protective of that.”

But, they added: “I think the rest of the family respects their charity work. They haven’t ever really put a foot wrong.”

With the double illnesses of this year, rarely has the course of the Royal family seemed so much in flux.

Buckingham Palace has been adamant that the King can and will continue his duties through cancer treatment, ably assisted by the Queen.

The Prince of Wales has decades of public service ahead, and an instinct to spare his wife any pressure as she recovers from illness. He has seen first-hand how his grandmother deployed her family troops, and enjoyed herself the more for having them there.

His willingness to include his cousins is one to watch. It was nice, said one person at the garden party, to see them having fun.