Image copyright Dan Dugdale Image caption Dan Dugdale contacted Sainsbury's after spending a "stressed evening at the vet's" with Suzie

Pet owners have complained to Sainsbury's after their dogs have fallen ill from eating promotional chocolate sent through the post.

The supermarket chain has posted free Green & Black's chocolate bars to its reward card holders in recent days.

Vets have warned that dogs only need to consume a very small amount of chocolate to find themselves in an "emergency situation".

Sainsbury's says it is investigating complaints "as a matter of urgency".

Sarah Hayward's cocker spaniel Jarvis was rushed to the vet after he tore into the promotional box while she was at work.

She told BBC Radio 5 Live: "My parents, who came home to let the dogs out at lunchtime, found the empty packet on his bed - and obviously they were trying to work out what it was initially.

"They realised it was chocolate and the second they called the vet they were told to rush him straight in.

"He was put on various drips to flush fluids down him to try and induce him to be sick and, yes, it was a bit of a worry".

Image copyright Sarah Hayward Image caption Sainsbury's has apologised to owners of the dogs affected, which include Jarvis

Meanwhile, Sammy Taylor, from Kent, wrote on Facebook that she was "furious" with the supermarket chain.

She said her puppy's stomach was pumped after it ate the chocolate.

Dan Dugdale, from York, said he contacted Sainsbury's after spending a "stressed evening at the vet's" with his dog.

He said that the actual reward card-holder did not live at his address.

A Sainsbury's spokesperson said the company was "extremely sorry for the distress caused".

"We know chocolate is unsafe for pets to eat and that's why we had measures in place to safeguard against pet owners receiving this promotion," they added.

Green & Black's declined to comment.

'Big threat'

Dr Huw Stacey, Director of Clinical Services at Vets4Pets, explained that chocolate can be toxic for dogs - especially dark chocolate as it contains higher levels of chocolate solids.

Chocolate contains theobromine, a naturally occurring chemical found in cocoa beans. It is fine for humans to eat, but dogs cannot metabolise it.

He told BBC Radio 5 Live: "It only takes a very small amount of dark chocolate to pose a big threat for dogs.

"The important thing is that the sooner you can do something about it, the better. If they've just eaten it you can give them drugs to make them vomit it back up again and get it out of their system".

According to the British Veterinary Association (BVA), chocolate can be poisonous for any animal but dogs are the most commonly affected pets:

They usually show signs of poisoning within 12 hours but symptoms can last for up to three days.

Symptoms include excessive thirst, vomiting, diarrhoea and restlessness.

They can develop into hyperactivity, tremors, abnormal heart rate, hyperthermia and rapid breathing.

In the worst cases, dogs can experience fits and in some cases can fall into a coma or die.

Owners who know their dogs have consumed chocolate are advised to call their vets "even if they look well in themselves", as the symptoms may not show up in the first hour or so, Dr Stacey said.