They were only telling the truth, but by the buttoned-up standards of the British Royals, it was quite an intervention.

The decision by Princes William and Harry to speak openly about the emotional chasm left by their mother's death attracted worldwide coverage and praise in almost equal measure.

The Windsors have hardly been a family to let it all hang out.

Yet just a few days ago, in a campaign designed to promote the mental health charity they support, the Princes admitted to the world that they had not spent enough time talking about Princess Diana since her death nearly 20 years ago.

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Harry confessed to feeling anger at the loss of his mother, while William explained to the television cameras that he has "uniquely bonded" with his brother through their shared grief.

They even went so far as to mention the C-word - counselling.

Yet amid the clamour of these latest media appearances, there was one rather important detail neither William nor Harry seemed quite willing to address.

Because, for all the heartfelt words on offer - about themselves, about their problems, about their late mother - there was no mention whatsoever of the man who devoted years to bringing them up as a single father. It is the other C-word - Charles.

"It's like an embarrassing omission in a wedding speech," as a senior source put it yesterday. "It's like the groom forgetting to thank the bride's father."

Or as another commented, in exasperation, it is as if Prince Charles has been "almost airbrushed out of the narrative".

For his own part, Charles has been characteristically magnanimous about this latest strange eclipse, if maybe just a "little perplexed".

He has grown used to rolling with the punches, allowing Princes William and Harry to develop at their own pace.

But if the heir to the throne has chosen to be generous, there are some who feel hurt on his behalf - and some who believe, more seriously, that William at the age of 34 and Harry, 32, remain a little self-absorbed.

With Charles taking on ever more duties from the Queen, it is now more than ever that he needs the support of his two sons.

In terms of TV audiences, web clicks and newspaper columns, their campaign for the Heads Together charity - which aims to end the stigma of mental illness -has been a triumph.

It gained global reach thanks to William's - rather stiff - web interview with American singer Lady Gaga, again on the subject of mental health.

And the drive culminates with yet more publicity today thanks to the London Marathon, which is promoting Heads Together as its official charity.

William's appearances on BBC TV and Radio 1 have been quite a coup for the Corporation - no wonder as the former BBC One Controller, Lorraine Heggessey, is the newly appointed head of the Princes' Royal Foundation.

Yet there are times when the Princes seem seduced by the easy appeal of their own publicity bandwagon and don't always pay heed to other Royal schedules which have been carefully coordinated far in advance.

Harry's plea to the media to take more care with its coverage of his girlfriend Meghan Markle, for example, proved a huge distraction from his father's visit to Oman, which was taking place at the same time. For William and Harry, this attitude is nothing new.

They were just 15 and 12 respectively when their mother died in August 1997, and a pattern was set.

Prince Charles put much of his personal life on hold following Diana's death. Photo / Getty Images
Prince Charles put much of his personal life on hold following Diana's death. Photo / Getty Images

One courtier recalls an episode which took place just a few years later in the Swiss ski resort of Klosters as Charles and the boys relaxed in their suite at the quietly luxurious Walserhof Hotel.

Not that there was anything quiet about that particular moment. Pop music thundered out as the Prince of Wales, then struggling as a single father, attempted to read in peace.

A senior aide walked into the room and politely asked if it were possible for all three Princes to pose together for a photo call the following morning.

The Prince turned to his sons, in hope. But all that followed from the two young men - still angry with the world, and with the media in particular - was a series of expletives.

The music was turned up even louder.

Clearly, it has not been easy for Charles yet, despite the tone of the past few days' coverage, those who know him acknowledge he has done everything he could to help his sons - and in trying circumstances.

He made sure he was available, for example, establishing a family rule that whenever Prince Harry was home from school, he, too, would be at home, even at the expense of official duties.

Following Diana's death, he reintroduced his former assistant Tiggy Legge-Bourke into the children's lives and encouraged the Princess's sisters, Lady Sarah McCorquodale and Lady Jane Fellowes, to play active roles in the boys' lives.

The wife of his late friend Hugh van Cutsem, Emilie, was also on the scene to provide much needed maternal comfort.

Meanwhile, his own personal life, including his relationship with the then Camilla Parker Bowles, had to be put on ice - in public at least, and certainly when he was with his sons.

Nor could he be accused of neglecting the psychological aspects of his children's upbringing.

His immediate and sure-footed reaction to Harry's brief experimentation with drugs was a case in point.

It was after a private party at Highgrove that a member of the Royal staff smelled cannabis and alerted Prince Charles - whose response was to take Harry to a drugs rehabilitation clinic in South London.

This was just for a day by way of a short, sharp shock, and it seemed to work.

As a teenager, Harry in particular, had struggled to cope with Diana's loss and admitted to trying cannabis on some occasions at Highgrove and drinking to excess at private parties and at a local pub in Wiltshire.

William, too, experienced understandable emotional problems following the disintegration of his parents' marriage and, of course, Diana's death.

Her tell-all Panorama interview in November 1995 left William perplexed and dismayed.
The Queen, meanwhile, encouraged William to make regular visits to Windsor Castle, just across the bridge from Eton, for tea and sympathy.

As one source close to Charles put it yesterday: "The Prince of Wales did his level best to be there for his sons throughout their formative years, in particular with the loss of their mother."

Another source, formerly with the Royal Household, said: "From the way Prince William and Prince Harry's mental health campaign has been presented and their discussions over their mother, it is as if Prince Charles was not part of the picture. Nothing could be farther than the truth. He was always there for them."

Not that they are always there for him in return.

Neither, for example, seems to have been willing to take over their father's charitable work, in particular The Prince's Trust, preferring to set up their own operation, The Royal Foundation.

Yet for his part, Charles manages to keep any frustration admirably in check and - however much he might feel pushed to the margins - has issued a statement in support of sons' and daughter-in-law's mental health drive.

After all, it is the Prince of Wales, more than anyone else in the Royal Family who has paved the way for a discussion of psychology - despite the famously stiff upper lip of his own father, Prince Philip.

It was Charles who had the honesty to admit that he had a thoroughly rotten time at boarding school, and Charles who took the rare step of admitting his own resort to therapy in the course of a difficult marriage.

In fact, whether he is supposed to be "talking to the plants" or discussing "holistic" wellbeing, it is Charles who for 40 years has been publicly mocked for daring to speak about the subject of mental health.

It seems a particular irony that he should be cut out of the discussion just now.