When the UK's Daily Mail, a loyal champion of the monarchy, splashes the headline "How did Kate photo become a PR disaster?", it is clear the royal family has a problem.

In an extraordinary episode for the family, Catherine, Princess of Wales, on Monday apologised and admitted to editing an official portrait of her released by the palace on Sunday. 

The edits had prompted AFP and other agencies to withdraw the altered image.

Kate, 42, has not been seen at a public event since attending a Christmas Day church service, and underwent abdominal surgery in January, fuelling speculation about her health.

The photograph dominated the front pages of UK newspapers landing on doormats on Monday, which had gone to print before the alterations were detected. 

The Sun tabloid ran with the headline "Looking Great, Kate", adding the picture "puts online rumours to bed".

But the rumour mill instead went into overdrive when the photographs were pulled late on Sunday.

Not only has the incident intensified speculation over Catherine's health but it has tested the bond of trust between the royal family and friendly media outlets.

Even the Press Association, which has among the closest working relationships with the monarchy, killed its distribution of the photograph.

Daily Mail columnist Richard Kay warned that "trust and integrity are precious commodities" in maintaining the public support that underpins the institution.

'Suspicions'

Royal-supporting tabloids would normally have "waited for it to blow over", Laura Clancy, lecturer in media at Lancaster University, told AFP.

Other outlets have also often been reluctant to dip into royal controversy over fear they could lose access, she added.

But the rise of social media now means that "people can question and talk about it and there's news outlets around the world who don't stick to that line".

In the digital media age, "any manipulation of an image, even relatively minor edits done with no intention to mislead, can raise suspicions", Chris Morris, chief executive of fact-checking service Full Fact, said in a comment sent to AFP.

Part of the problem is that the family distributes pictures from self-described "amateur photographer" Catherine for use by professional media outlets, which have strict rules on using manipulated images.

While there have been calls to use professionals, "there are some ... individuals whose brand is based on authenticity", explained Hannah Perry, lead digital researcher at the London-based think tank Demos. 

But royals must weigh that up against the fact that "trust is so low in public institutions and we know that people are becoming incredibly sceptical and also savvy" about analysing information, she added.

"The best-case scenario in that situation would be to be transparent," suggested Perry.

On the streets of London, public reaction was mixed.

'Obsessive need for secrecy'

"I was actually quite shocked. I would think it would be a lot more seamless coming from such an official source," said 21-year-old student Flora Canavan.

She added that her trust in the royal family had not been eroded as "I don't think I trusted them beforehand".

However, solicitor Jen Chambers said the incident had been "massively blown up out of proportion" and that "the kids probably weren't pulling good faces in the photo".

Although the furore has caused embarrassment for loyal media outlets, they still remain supportive, for now.

The Sun's front page on Tuesday urged "social media trolls, idiotic conspiracy theorists and sniping media critics" to "lay off Kate".

It accused critics of waging "a bullying campaign against a devoted mum, recovering from a serious operation, who simply wanted to offer the public a perfect portrait of her and her kids".

While the Daily Mail's Kay said it was "easy to see why the couple acted as they did", he offered a stark warning. 

"They are down to earth and unshowy. But it is this desire for ordinariness which conceals one significant flaw: a near obsessive need for secrecy.

"If pictures can be digitally altered, what else can be twisted? The British public adore the Royal Family but that adoration rests on them being told the truth.

"There are precious commodities at stake here: trust and integrity."