Graham Smith, CEO of Republic, says he "does not buy" the apology from the Princess of Wales, adding it "doesn't explain anything".
The leader of the anti-monarchist group said that the said the couple need to think more about how they pass on information to the public.
It comes after a storm surrounding the first image of Kate to be released since her abdominal surgery in January.
The Princess was forced to apologise for any “confusion” caused by a botched edit and claimed she was just "experimenting" with her photoshop skills.
Graham Smith said it ‘has all the credibility of Andrew’s Newsnight interview’
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Smith said: "It doesn't really explain anything. We could see it's been edited.
"The question is why? Why haven't they shown us the original photo? Because it's not just edited in a way that she's trying to tidy it up.
"It looks photoshopped in a way that make you question were those people all in the photo when it was taken?
"That's what it looks like. I'm not saying they weren't or were, It's very odd and I don't think people are going to be convinced."
The future Queen issued an apology for the photoshop blunders
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He added: “It has been long complained by many journalists that they don’t allow press photographers and they have to rely on in-house photographers.
“That has to stop, for their own benefit. Otherwise, nobody will believe what they put out from now on.”
LATEST ROYAL NEWS:
Kate apologised in a statement released on social media: “Like many amateur photographers, I do occasionally experiment with editing.
“I wanted to express my apologies for any confusion the family photograph we shared yesterday caused.
The photograph of Kate and her children, taken by William, was withdrawn
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“I hope everyone celebrating had a very happy Mother’s Day. C.”
The photograph of Kate and her children, taken by William, was withdrawn with a ‘Kill’ notice by international picture agencies hours after its release.
Royal sources said the Princess of Wales made “minor adjustments” and that Kate and the Prince of Wales wanted to offer an informal picture of the family together for Mother’s Day.
“This was an amateur, family photograph taken by the Prince of Wales. Their Royal Highnesses wanted to offer an informal picture of the family together for Mother’s Day,” the source said.
“The Princess made minor adjustments as she shared in her statement on social media, the Wales family spent Mother’s Day together and had a wonderful day.”