The pressure and speculation about the Princess of Wales’s health before she disclosed her cancer diagnosis was “the worst I’ve ever seen”, one of her former advisers has said.

Catherine, 42, said in a video message on Friday that she was having “preventative chemotherapy” after major abdominal surgery in January.

The statement followed weeks of frenzied rumour and conspiracy theory on both social media and in traditional media outlets.

Paddy Harverson, who was previously the official spokesperson of Kate and her husband, the Prince of Wales, said the speculation had reached new levels before Friday’s announcement, which has prompted a global outpouring of support.

Appearing on BBC One’s Sunday With Laura Kuenssberg, Harverson was asked whether social media or the mainstream media bore most responsibility for the pressure on Catherine.

He said: “Well, it feeds off itself … It’s a sort of permanent doom loop. And it’s the worst I’ve ever seen.”

Harverson, who was also communication secretary to the king when he was the Prince of Wales, said he believed the royal family would have chosen to make the announcement in the same way even without the pressure.

“I’m absolutely convinced that if we hadn’t had all the madness and social media, if we hadn’t had the Mother’s Day photo mistake, they would have still done it like this,” he said. “They would have still waited till this last Friday when the schools are breaking up to make the announcement.”

Catherine and William said on Sunday they had been “enormously touched” by the outpouring of support from all over the world.

Harverson denied that the institution had been left “fragile” now that King Charles III and Catherine were receiving cancer treatment. “We just have to come to terms with the new reality, there’s fewer of them,” he said.

“So, everyone just needs to sort of understand that they will still be busy, they will get over this. I am highly confident that the king, who I know well, is incredibly strong, very resilient, a great spiritual person, so I know he’ll get through it.

“Likewise with the princess, with Kate, I think once they’re through this sticky patch, I think we’ll get back to normal. I think the nation just needs to adjust, and the media, and everyone who follows this, to a smaller family but still very busy doing what they do in their own way.”

Jeremy Hunt, the chancellor, on Sunday praised Catherine and the king. He also told Laura Kuenssberg: “My thoughts are also with the Princess of Wales and the king, and how much I think we all admire their incredible stoicism.”

Anneliese Dodds, the Labour party chair, said she felt a “huge amount of sympathy” for the princess while watching her announce her cancer diagnosis on Friday evening.

Rumours online about the princess intensified after William cancelled at short notice his appearance at a memorial service for his godfather, the late King Constantine of Greece, on 27 February.

It is now understood the prince’s decision not to attend was related to his wife’s condition but it is not known when the princess was told about the cancer.

Further speculation was triggered after the princess admitted to “editing” her official Mother’s Day photograph, which had irregularities.

In a further development, an investigation was launched by the UK’s privacy watchdog, the Information Commissioner’s Office, into allegations there were attempts to access Kate’s private medical records at the London Clinic, where she had her surgery.

It is not known how long Catherine will be receiving treatment but it is understood she may be keen to attend events as and when she feels able to, in line with medical advice, although this will not indicate a return to full-time duties.

William will continue to balance his official duties with caring for the family as he has done since her operation. He is due to return to public duties after his children return to school after the Easter break.

He and his wife will not attend the royal family’s traditional Easter Sunday service at St George’s Chapel at Windsor Castle, which the king is hoping to go to with the Queen if his health allows.

It is not likely to be a large family gathering or service, according to The Telegraph, as the king has paused public-facing royal duties.

Charles has been receiving treatment for an undisclosed form of cancer since early February after he was treated for an enlarged prostate at the same hospital where Kate had abdominal surgery.

The king’s nephew Peter Phillips said on Sunday that Charles was “frustrated” that his recovery was taking longer than “he would want it to”. He added that the king was in good spirits and was pushing his staff to be able to return to his duties after beginning treatment for cancer last month.

Phillips, the son of Princess Anne and Capt Mark Phillips, told Sky News Australia: “I think ultimately he’s hugely frustrated. He’s frustrated that he can’t get on and do everything that he wants to be able to do. But he is very pragmatic, he understands that there’s a period of time that he really needs to focus on himself.

“But at the same time he is always pushing his staff and everybody – his doctors and nurses – to be able to say: ‘Actually can I do this, can I do that?’”