The serial murderer, who raped and strangled two teenage girls after a night out at Edinburgh’s World’s End pub in 1977, cannot walk and is unable to eat solid food after a series of strokes.
The “skeletal” 72-year-old spends his time in bed or a wheelchair, and is surviving on a liquid diet fed from a baby cup.
A prison insider says his condition has gone downhill rapidly and he is showing no signs of getting better.
Sinclair is one of the country’s longest-serving prisoners, having been behind bars since 1982.
He is serving life sentences for the murders of Helen Scott and Christine Eadie, both 17, in the capital and Mary Gallacher, also 17, in Glasgow.
He had previously been jailed for killing eight-year-old Catherine Reehill in Glasgow when he was aged just 16.
But he is also suspected of murdering Frances Barker, 37, Hilda McAuley, 36, Agnes Cooney, 23, and Anna Kenny, 20, all in Glasgow.
Sinclair has suffered a number of strokes in the past three years and is under constant medical supervision in Glenochil prison, near Alloa in Clackmannanshire.
“Sinclair is now skin and bones,” the source said.
“He can’t walk from his bed to the toilet unaided. He used to have a walking frame but now has a wheelchair.
“He knows when he’s being spoken to, and it’s believed he knows what’s being asked of him, but that’s about it – he’s incapable of doing most things.
“Sinclair has an alarm on his wrist which he can activate if he falls over or wants assistance.
“People do not relish having to attend to him but they have a duty of care. He still has an evil look in his eyes.”
In 2014 Sinclair became the first person in Scotland to be retried for the same offence after previously being acquitted of the World’s End murders.
He was sentenced to 37 years in prison, the longest term in Scottish legal history, and will not be eligible for parole until he is 106.
An appeal against the sentence was rejected in 2016.
“Sinclair is now deteriorating rapidly,” the source said.
“He eats only wet food, drinks from a plastic cup like a baby’s bottle and is spoon-fed his meds.
“It would be a tragic thing to see if it was someone other than him.
“Nurses visit three times daily and he’ll be lucky to see the end of the year.”