Police have issued a fresh appeal for information on the disappearance of Sandy Davidson, who was three years old when he vanished 40 years ago.

Sandy had been playing in his grandparents' garden in Irvine when the gate flew open and their dog escaped on the morning of April 23, 1976.

He chased after the dog and despite extensive searches he was never seen again. He was a few weeks short of his fourth birthday at the time.

On the 40th anniversary of his disappearance, the charity Missing People say his family has been living in "limbo" ever since.

Images of Sandy will be shown on digital billboards and online as Police Scotland attempt to uncover new information about the case.

An "age-processed" image, which gives an impression of what he may look like now, has also been produced.

His sister Donna, who was playing with him in the garden in St Kilda Street before the three-year-old vanished, has worked tirelessly to try and find her brother.

Speaking exclusively to STV News, Donna, now 42, said: "I don't think he's still alive, but we still need to find him. We've never had a funeral or a memorial service and we never will until he's found.

"I think someone reached over the fence and opened the garden gate from the outside, which let the dog out and sent us after it.

"Somebody knows something. We've lived with this for so long - it's taken over our lives."

Detective superintendent David Halliday, the senior investigating officer, said: "It's hard to imagine the distress and sadness Sandy's family have endured over the last forty years, not knowing what has happened to their beloved son and brother, who was only a toddler when he went missing.

"Despite the passage of time, this missing person investigation remains open and I'd like to take this opportunity on the anniversary of Sandy's disappearance to ask people to cast their minds back to Friday, April 23, 1976.

"Did you live in Bourtreehill in Irvine, specifically around the area where Sandy was last seen in St Kilda Street?

"April 23 was a relatively warm day as Sandy played outside with his sister Donna. The community of Bourtreehill was tight knit and mostly everyone knew their neighbour.

"Did you see Sandy when he left his grandmother's garden? He was quite a distinctive looking child with light blonde hair and blue eyes.

"I would like to encourage anyone with information that may assist in establishing the circumstances of Sandy's disappearance and provide new lines of enquiry to contact Police Scotland on 101."

Jo Youle, chief executive of the charity Missing People, said: "To spend any length of time with a loved one missing is heart breaking for a family desperate for news.

"Sandy's family have had to endure an unimaginable 40 years since Sandy disappeared.

"Everyone at the charity joins Police Scotland and the rest of the public in the hope that this new appeal will finally end the limbo that Sandy's family has been living in since the day he disappeared."