Cheryl Cole fought for custody of chihuahuas Buster and Coco when she divorced her husband Ashley Cole.

London - The rattle of the letter-box heralded the arrival of another official-looking envelope on Lisa Olson’s doormat. She assumed it was another solicitor’s letter but was not especially fazed, since the division of assets between her and her estranged husband was unlikely to be complicated.

They had been married only a year when her husband packed his bags and left. The sorting out of books, CDs and mementoes was a perfunctory process; they had been renting a flat and, after such a short time together, there were few shared possessions.

But as Lisa opened the envelope and read its contents, her heart sank. Peter wanted the one thing that bonded them emotionally, the only thing that had made them - albeit briefly - a family.

Her ex wasn’t about to fight his wife for custody of a much-loved child. What Peter wanted was Blackie - their cat.

Lisa, 41, a creative artist from Sutton Coldfield in the West Midlands, says: “I was flabbergasted. He said he’d become ‘very fond’ of the cat and wanted custody, but I felt what he really wanted was to hurt me. I could see this was going to be a bitter and expensive legal wrangle, but I couldn’t bear to lose. Blackie meant the world to me.

“What hurt me most was that Blackie was my cat; he’d been lent to me by an aunt years before I’d even met Peter. My aunt was ill and felt she couldn’t look after the cat properly. He was just a moggy, with no financial value whatsoever, but he was beautiful, and I loved him like a surrogate child.

“Blackie always chose my lap to curl up on in the evening. Blackie didn’t love Peter as he loved me. I immediately instructed my solicitor to reply that Peter absolutely could NOT have the cat,” she says.

The battle lines were drawn though, thankfully, the fight itself was short-lived. In fact, it was settled within a few months, without the couple having to take their case to court. Even so, it added another £1,000 to Lisa’s legal bill.

She says: “In the end, it came down to ownership; my solicitor argued, successfully, that my aunt was Blackie’s legal owner and that he was only on loan to me.

“I was happy with the outcome, but getting there was almost as bad as the breakdown of the marriage itself,” she says. “It was bizarre; I’d heard of couples fighting over custody of a child, but never an animal.”

However, Lisa’s legal wrangle over Blackie is far from unique. A surveysays custody battles over pets are becoming as expensive and emotionally-charged as those over children, with 20 percent of warring couples fighting over their animals.

Joint custody agreements are also becoming more common, with 20 percent of separating couples drawing up formal access arrangements for their pets.

The survey, carried out by The Co-operative Pet Insurance, found one in ten couples described the pain of losing a pet as more acute than losing their partner.

And, as is often the case in the world of celebrities, where money traditionally trumps good sense, there has been a spate of high-profile court cases involving couples wrangling over a shared pet.

The most recent involved Cheryl Cole, who fought for custody of chihuahuas Buster and Coco when she divorced her husband Ashley Cole last September. It was rumoured that Ashley eventually settled for access visits.

Perhaps the most bitter case was that of former Big Brother contestants Melanie Hill and Alex Sibley and their fight over Staffordshire bull terrier Poppy, a £70 rescue dog they had bought during their three-year relationship.

The case culminated in court in 2007, where a joint custody arrangement was agreed in which Alex got the dog for two weeks out of every five. The legal bill? A reported - and staggering - £25,000.

Staking everything on a pet is something mom-of-three Nicola Smith, from Wishaw, in Lanarkshire, understands.

The 32-year-old was sued by her ex-boyfriend, Kevin Holmes, for custody of a parrot called Gaz, after they split up last year. It was an ordeal Nicola describes as “absolutely terrifying”.

“But there was no way I was going to let Gaz go without a fight,” she says.

Nicola had bought the parrot for £240 from Kevin’s cousin in 2004. “He was a fantastic bird. He grew particularly fond of my ten-year-old daughter, Shannon. He’d sit and watch her playing in the garden. He was happiest sitting with us on the sofa, watching EastEnders (he loved the theme tune) as he nibbled on a cracker.”

When Nicola’s relationship with Kevin floundered, the one thing he insisted on was custody of the parrot. “He said the bird was his, as he’d been bought from his cousin, even though the money had come from my savings.

“But that wasn’t the point. Gaz seemed to hate Kevin - he was always teasing him. The kids loved Gaz so much, they would have been devastated to lose him. I couldn’t let that happen.”

The court eventually found in Nicola’s favour. “Lots of people couldn’t understand why I was putting myself through so much stress for a bird, but they didn’t understand our attachment to him,” she says.

“He wasn’t just a pet, he was a member of the family. I knew I could have been liable to pay the court fees if I’d lost, but it was a risk I had to take.”

As zealous as Britons can be when it comes to custody of their pets, it seems the Americans still trump us. Family courts in the US have heard depositions from divorcees relating to parakeets, horses, monkeys, tarantulas and, in one instance, a frog, which showed its disdain for the drawn-out proceedings by dying mid-case.

Growing fat on the proceeds are the lawyers. Several legal firms in the US earn significant sums from negotiating animal pre-nups, the drafting of visitation rights, and pet-related financial bequests. A new breed of specialist “pet lawyer” has emerged who advertise their services using slogans such as “You Get the Car, I Get the Cat”.

So is the UK heading down the same path? Well, lawyers say we’re not far off. Martin Loxley, head of family law at the Sheffield-based solicitor Irwin Mitchell, says: “Pet custody cases are still rare in this country, but they do happen.

“They usually involve childless couples for whom the pet has become a surrogate. A lot of tears have been spilt in our offices over dogs and cats.”

Martin recalls the case of a couple who signed a legal agreement to share two Labradors after their divorce.

The dogs now swap homes every weekend, in an arrangement which has worked successfully for several years.

More often, however, couples fail to reach a compromise, and the animal ends up in limbo. London’s Battersea Dogs Home cites divorce as the most common reason for animals being left at the shelter.

Such was the case with Hayley Unsworth, a 28-year-old lettings manager from Altrincham, in Cheshire, who bought a black Labrador puppy, Paddy, during her seven-year relationship with her ex-fiance.

“We thought long and hard about buying a dog, but not about what would happen should we break up. It’s something I regret terribly,” she says.

They split up in 2007 and Hayley moved out of their three-bedroom house into a flat, which was unsuitable for a dog. “My ex took the dog with him when he moved in with his dad while we sold the house. I assumed it was a temporary arrangement until I found a more suitable property.”

Unbeknown to Hayley, her ex arranged for Paddy to go to a dog rescue centre, from where he was placed in a new home.

“My ex broke this news to me in a very matter-of-fact way a few weeks later, while we were talking through the papers relating to the sale of the house,” Hayley says.

“I couldn’t speak, I was so upset.

“I still think about Paddy every day, and have a photo of him by my bed. Whenever I see a black Labrador playing with a family on a beach, I wonder if it’s Paddy.

“I can understand couples who spend thousands of pounds fighting for a pet but unfortunately it’s too late for me. I’ve lost Paddy forever.” - Daily Mail

* Additional reporting by Diana Appleyard