Two nine-week-old pug puppies from Lithuania have been taken into quarantine by trading standards officers in Northamptonshire.
County council officers were called by a vet in Corby after he became concerned the puppies had not been vaccinated for rabies.
The animals had papers showing they had come from Lithuania.
Trading standards officers believe the dogs were brought into the UK illegally and must be quarantined to stay.
For dogs entering the UK, it is necessary for them to spend six months in an approved quarantine kennels or to have complied with the requirements of the Pet Travel Scheme (PETS).
To enter the UK without quarantine from a country listed in the scheme's regulations - which includes Lithuania - a pet must have been micro-chipped, vaccinated against rabies and blood-tested.
The owners of the dogs will now be asked to pay for them to be quarantined or fund their return to Lithuania.
Andre Gonzalez de Savage, council cabinet member for "customers and communities", said in a statement: "It appears that the two puppies arrived in this country illegally, breaching regulations that are in place to safeguard against rabies and other diseases.
"I am pleased that trading standards were able to act so quickly to take both the dogs into quarantine.
"Public health is a key priority for our county and therefore we take any infringement very seriously and will take the appropriate action as quickly as possible."
Full details of requirements for pets entering and leaving the UK are on the Department for Environment, Food and Rural Affairs (Defra) website.