The organisation has not allowed Marie Davidson, 48, to see her animals, or even told her where they have been kept for the past three months.
The only correspondence she has had from the society has been two letters, both received in the last fortnight – one to say that its investigation is continuing and another detailing its complaints procedure.
The case has led critics to attack the RSPCA's tactics.
Early 2008, an RSPCA inspector called at Miss Davidson's home in Hatfield, Hertfordshire, after an anonymous phone call about the animals.
The official carried out an assessment of the dogs, Rocky and Chubby, and said they were overweight. The inspector gave advice on how their weight could be reduced.
Miss Davidson, who works for Hertfordshire social services and who has owned dogs for more than 20 years, followed the instructions and managed to bring down their weight.
However, she had to cancel three vets appointments to have her pets weighed, after her mother fell ill and she had to care for her.
"There was a dramatic weight loss," she said. "It was working, although I admit I did lapse for a bit and start to give them a few titbits again."
After the missed appointments, an RSPCA inspector returned to her house in October while Miss Davidson was at work. The inspector asked her partner, Terry Shadbolt, for permission to take the animals to the vets.
"The inspector just asked if she could take them to get them weighed, and he said yes," said Miss Davidson. "When I got home, I rang the vets, and the RSPCA inspector said that under the guidance of the vet, the dogs were not coming home.
"Three days later, the inspector came to interview me. But since then, I haven't been able to find out anything about the dogs.
"I am absolutely distraught. I have been put on antidepressants as a result of this.
"I do realise it was not good to let them get so big. But I love those dogs. At no point did anyone, the vet or the RSPCA, tell me about the devastating consequences of the dogs not losing weight. I was doing my best to sort the problem out."
Miss Davidson is registered disabled. Although she can walk, she cannot take the dogs on long walks.
"They could have suggested I get a dog walker to help me, and I would have done," she added. "But they suddenly seized the animals without warning."
When the dogs, who are three and half years old, were initially inspected, Rocky weighed 73kg and Chubby 60kg, according to Miss Davidson.
She claimed she had managed to get Rocky down to just over 50kg, and Chubby to about 46kg, although their weights had increased again.
According to the RSPCA, when they were taken, Rocky weighed 63kg and Chubby 52kg. The organisation says a healthy Labrador should weigh no more than 34kg.
Research last year revealed that around two million dogs in Britain are overweight.
An RSPCA spokeswoman said: "It is understood that the two dogs were removed with the owners full consent and were taken to the vets by the RSPCA for an appointment.
"Following an examination of the animals, the independent vet contacted the police, who after viewing the dogs and listening to the veterinary advice, made the decision to seize the animals under the Animal Welfare Act 2006.
"Miss Davidson's file is currently under consideration by the RSPCA's prosecution department and a decision is expected in the near future."
Hertfordshire police confirmed a constable had authorised the seizure.
If Miss Davidson is prosecuted under the Animal Welfare Act she faces a fine, a ban on keeping animals, or even a jail term. If she is not, and the dogs are returned, she could be asked to reimburse the RSPCA for the cost of keeping them in kennels.
Frank Field, the Labour MP, said: "These tactics are the exact opposite of what most animal lovers think the RSPCA does. I can't imagine that these dogs are better off in RSPCA kennels than in a loving home.
"I'm planning to stage a debate as to whether the RSPCA should actually have its prosecution powers rescinded. I don't think that when Parliament passed them it ever imagined that things like this would be happening."
A representative of the vets' surgery said they had been instructed not to comment by the RSPCA.