After the Princess of Wales announced on Friday that she is undergoing treatment for cancer, the US actor Blake Lively apologized to Catherine for joking about a manipulated family photograph that the latter recently published as speculation about her whereabouts ran rampant.

“I’m sure no one cares today but I feel like I have to acknowledge this. I made a silly post around the ‘Photoshop fails’ frenzy, and oh man, that post has me mortified today,” Lively wrote on Instagram. The star of Gossip Girl and A Simple Favor added: “I’m sorry. Sending love and well wishes to all, always.”

Lively, who is married to actor and Wrexham AFC co-owner Ryan Reynolds, also deleted photos of herself promoting certain drinks that were badly edited and captioned: “Now you know why I’ve been [missing in action].”

The post taken down by Lively came shortly after Catherine marked Mother’s Day in the UK with a photo of her with her children – George 10, Charlotte, eight, and Louis, five – that major news organizations recalled from distribution, claiming it had been altered. After the photograph ignited baseless speculation about why Catherine had been keeping a low profile most of this year, Catherine apologized for any “confusion” created by the image and said she had digitally edited it herself.

Lively’s apology came as other fellow American entertainers are now facing pressure to apologize for making jokes at Catherine’s expense before she revealed on Friday that she has been out of the public eye as a result of cancer that was discovered after she underwent abdominal surgery in January.

Among them was reality television star Kim Kardashian, who wrote “On my way to go find Kate” on an Instagram post published five days before Catherine disclosed her ongoing cancer battle. “I think an apology is needed,” read a reply to the post that was left on Friday.

Meanwhile, Late Show host Stephen Colbert reportedly earned a notice threatening legal action after the comedian joked about another unsubstantiated rumor: that Catherine’s absence of late perhaps stemmed from philandering by her husband, William, the Prince of Wales. Instagram users flooded Colbert’s Instagram page with comments such as “Well done for making fun of a woman with cancer”, “shame on you” and “you bullied a woman with cancer”.

Catherine’s announcement on Friday came in a video message recorded in the gardens of Windsor Castle. She said she had withdrawn from public recently as she and her husband had required time to explain her health situation to their sons and daughter.

“We hope you will understand that, as a family, we now need some time, space and privacy while I complete my treatment,” she said in the video, along with other remarks. “I am well and getting stronger every day by focusing on the things that will help me heal – in my mind, body and spirits.”

The Press Association contributed reporting

• Catherine, Princess of Wales
• Cancer
• Health
• news