Last week in Belfast, the highest court in the land rejected the final appeal for a pet dog called Lennox to be saved. Only an appeal on a point of law in the next four weeks – or a pardon – can save him now.

If Lennox was a human, Amnesty International would have him as a poster boy. He was an adorable puppy, as you can see from the photos. His owner, Caroline Barnes, a former vet nurse, thought he was probably a mix of American bulldog, labrador and staffie – as a crossbreed, she didn't know how big he would grow. For his first five years, he was an adored family pet.

Then, one day in May 2010, he was placed on death row. He hasn't been seen by his owners since. He had done nothing wrong. Under Northern Ireland law, it is illegal to own a dog that looks like a pit bull and they can only be saved from execution if a judge rules they are "satisfied that the dog will not be a danger to the public". But this exemption is hardly ever used – to date, only one dog that I can find has survived being identified as a "pit bull type" in Northern Ireland.

In England, Scotland and Wales our unpopular and ineffective Dangerous Dogs Act was amended in 1997 to remove the mandatory death sentence for any dogs identified as pit bulls. Northern Ireland's devolution meant they no longer followed our dog laws, and they had to wait until 2011 and the Dogs (Amendment) Act (Northern Ireland) 2011 for well-behaved pit bull types to be given even the slightest chance of avoiding a death sentence.

Lennox in his cell

And so it was that a tape measure "proved" Lennox was a pit bull, effectively sentencing him to death. The dog warden measured the length of his legs and the width of his muzzle. These dimensions and ratios could fit many dogs that are definitely not genetically a pit bull. For example, a pit bull is said to have a femur that is shorter than its tibia. The court does not have to prove that the dog is definitely a pit bull, the burden of proof is on the owner to prove that their dog is not.

So was Lennox a danger to the public? He was always walked on a lead. The family's garden was well fenced, so he never roamed. He had an up-to-date dog licence. He was neutered, insured and microchipped. He was even an unofficial assistance dog for the family's 11-year-old daughter Brooke, who is registered disabled.

Three quite conflicting witnesses have reported on Lennox's behaviour, the judge preferring the most negative views. The most positive was dismissed out of hand.

Lennox allegedly jumped up at one of the three dog wardens who took him away from his family. As the dog trainer Victoria Stilwell says: "If all dogs who jump up on strangers in their house were guilty of being dangerous dogs, there would not be many dogs left in homes." But even the jumped-up jumping up charge is in dispute. I'm told by a family friend that Lennox actually tried to climb onto Caroline's lap "like a little child" to get away from the dog wardens.

The dog wardens claimed in court that they had been warned by an unnamed man at the house that Lennox could "rip their head off". That Lennox had lunged at one of them and hit her with his muzzle. The family deny these allegations – it's a classic "he said, she said". But before that day in 2010, the record shows that no one had ever complained about Lennox.

When 11-year-old Brooke first heard that her dog had been taken away she had a very serious asthma attack. For the last two years the family – who are new to being on the wrong side of the law – have devoted all their energies into fighting for Lennox's life. Almost 150,000 people have now signed their petition – almost a quarter of the population of Belfast. Some more radical supporters have gone further, starting a "boycott Belfast" campaign to shame the city into action.

Stilwell is not the only dog expert to back the campaign to free Lennox. Jim Crosby is an ex-cop who has assessed more than 30 dogs that have actually killed people. No one else on earth has as much experience of dangerous dogs as him. He has told the Northern Ireland first minister, Peter Robinson, that he is willing to fly over to Northern Ireland to assess Lennox. He hasn't yet had a reply, despite Robinson saying how much he disagrees with what is happening to Lennox.

Lennox has lost a lot of fur in the past two years

What does Crosby think about using a tape measure to assess whether a dog is dangerous? "I have seen dangerous dogs that looked like pit bulls, and like huskies, and rottweilers, and chihuahuas, and labradors … and on it goes. [Dangerous dogs] have as many faces as human killers. Yet they all have one thing in common: they have exhibited observed behaviour that shows they present a clear danger, or have behaved in a way that caused serious injury or death.

"I can't honestly say that I would have given Lennox a clear pass; that I never know until after I work with a dog, one on one. But Lennox would have got a fair evaluation, documenting the behaviour seen and quantified, from square one. The size of his head would not have mattered – it would have been the size of his heart that counted."

Being in Belfast city council's "care" may have driven the poor dog mad (he has lost a lot of fur, almost certainly due to stress) in which case it could be seen as a merciful release to see him put to rest. But if there is even a chance that Lennox is still the same good dog that loved his family, that has pined for them these last two years. Surely Crosby should be allowed to apply his years of experience to Lennox's case?

But the fact is, without an intervention from either the Queen or the secretary of state for Northern Ireland – or a last-minute point of law being found by the overworked legal aid team – Lennox will be killed within a month.

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