Former Home Secretary and Sheffield MP David Blunkett may be without a guide dog for six month following the death of his sixth dog last week.
Cosby, a seven-year-old Labrador-retriever cross was put down last Friday following the discovery of an inoperable liver tumour.
He was Lord Blunkett's sixth guide dog - following Ruby, Teddy, Offa, Lucy and Sadie - to have assisted him since he qualified to use one in 1969.
His unexpected death means Lord Blunkett may have to wait around six months to be matched with a new dog to help him get around Westminster and the Peak District countryside around his Derbyshire home.
Lord Blunkett described the decision to put the 'big, lovable and endearing' dog to sleep as 'heartbreaking'.
"Such a moment would be upsetting for any dog-owner, but even for a hard-bitten ex-Home Secretary, this was a deeply harrowing decision," he said.
"What made it more traumatic was the speed with which events unfolded. The previous Sunday, in Sheffield, had seen him chasing a ball and being his usual exuberant self."
Despite being spooked by Guy Fawkes fireworks over the weekend, Cosby guided the Labour peer round the Palace of Westminster 'without a grumble' on Monday, and it was only when he went off his food that Lord Blunkett realised something was wrong and took him to the vet.
"How I will miss Cosby's ever-wagging tail," said Lord Blunkett.
"Above all, I shall remember his incredible bravery in those last few days, including walking unaided into the veterinary hospital.
"And, even at the last, giving us a wag of his tail as he sniffed one of his favourite tennis balls, which smelt of the woods where he spent many happy hours.
"So thank you, not so old friend. I shall miss you, and so will many others."