A police investigation has been launched after a woman was hit by a royal convoy.
The collision is believed to have involved a police motorcycle and happened as the Duke and Duchess of Cambridge made their way from London to Windsor.
The woman, named locally as Irene Mayor, was taken to hospital in a critical condition, Scotland Yard said.
William and Kate were "deeply concerned and saddened" by the event and had been in touch with the elderly woman's family and sent flowers, a spokesperson told the Daily Mail.
Louise Rawlinson, 41, helped tend to Ms Mayor until paramedics arrived.
She had been moving furniture out of her shop when a passing police bike made a noise, prompting her to look around and see the pensioner on the floor.
The pensioner had shopping with her and seemed like she "did not realise what had happened", according to Ms Rawlinson.
She said that Ms Mayor had an injury to the back of her head which was bleeding heavily, which she held and treated.
Ms Rawlinson said: "She was talking, I wasn't concerned let's just put it that way, because she was speaking."
The officer had not come off his bike during the incident and had "clipped" Ms Mayor, according to Ms Rawlinson. She also described the officer as being visibly upset and looking "grey".
She said there were two officers and others helping the woman.
Ms Rawlinson said: "He was there the whole time, the whole time. He was taking her pulse, he was amazing. We all worked as a team.
"They did a fantastic job, and I felt that they were really professional and he wasn't going fast."
Ms Rawlinson said she overheard paramedics at the scene say that Ms Mayor had been clipped by a wing mirror.
"It was an accident, and I think what it was they just did not see each other," she said.
A witness told The Sun: "His bike hit her and she spun round, off her feet and fell badly on the floor."
Her daughter Fiona told the newspaper the royal couple have offered to visit.
She added: "She's got lots of injuries. They are just keeping her stable at the moment."
Another witness, who did not want to give his name, told the Press Association he saw a woman lying in the road in the aftermath of the accident when he opened one of his windows.
The 76-year-old said: "There were people around her trying to make her comfortable – bringing blankets and putting a pillow under her head, that kind of thing.
"Everyone was talking to her so she was awake. I don't think her legs were moving at all but her arms were moving slightly.
"I saw a police bike... and it was on its side, on the wrong side of the road facing the wrong way."
An IOPC spokesman said: "The woman, in her 80s, suffered serious injuries and was taken to a London hospital where she remains in a serious but stable condition.
"In line with procedure, the Metropolitan Police Service referred the collision to the IOPC.
"Our staff attended the scene of the incident and after careful consideration, we have launched an independent investigation.
"The investigation is in its very early stages and the officer involved is assisting our inquiries as a witness.
"Our immediate thoughts are with the injured woman and her family and those affected by the incident."
Kensington Palace said: "The Duke and Duchess of Cambridge were deeply concerned and saddened to hear about the accident on Monday afternoon.
"Their Royal Highnesses have sent their very best wishes to Irene and her family and will stay in touch throughout every stage of her recovery."
The accident took place at around 12.50pm on Monday when the royal couple were on their way to Windsor for the St George's Chapel service commemorating the Order of the Garter.
Giles Oakley, a neighbour who lives opposite Ms Mayor, told the Press Association: "They're a lovely family.
"It is very, very sad that Irene has been injured. She is a very sweet person and is very well liked in the street."
Earlier this year, the Duke of Edinburgh, 98, surrendered his driving licence after a crash at Sandringham.
Philip flipped his Land Rover Freelander on January 17 after colliding with a Kia as he pulled out onto the A149 in Norfolk.