People who bought puppies during coronavirus lockdowns have been warned to watch out for potentially dangerous behaviour changes.
The pandemic saw a surge in new dog owners in the UK, as people sought companionship in lockdown.
Some dogs have had behaviour problems - particularly after being left alone for the first time when their owners returned to work.
The RSPCA fears the problems could get worse as the puppies reach maturity.
Dr Samantha Gaines, head of companion animals at the charity, told MPs: "Operationally, we are seeing an increase in the more challenging dogs that are coming into our care, which we think is an impact of Covid."
Many "pandemic puppies" were imported from outside the UK and had not been bred or reared in an appropriate way, she told the Commons Environment committee.
Growing up in lockdown meant they were not socialised properly, with fewer people visiting their home, according to Royal Veterinary College (RVC) research on the early lives of puppies purchased in 2020.
'Fears and phobias'
Some had not developed into well-balanced adult dogs and had suffered "separation anxiety" at the end of lockdown, resulting in destructive behaviour.
Dr Gaines said she had seen new data from the RVC study, due to be published later this year, which suggests behaviour problems were only now beginning to appear in some dogs.
"Based on the research we probably shouldn't think that we have actually seen the worst of it," she told MPs, adding that dog behaviour often changed "between 2.5 and 3.7 years of age".
"And bearing in mind that the pandemic only happened in 2020 we probably still have some dogs who are developing and are yet to present those behaviour problems," she told the MPs.
Speaking after the hearing to the BBC, Dr Gaines said dogs who had not been socialised properly could develop "certain fears and phobias".
Owners also needed to be aware that aggression "can manifest in dogs that are frightened or anxious, so there is that potential risk," she added.
She advised anyone who had seen sudden behaviour changes in their dog to go to a vet, who will be able to rule out any medical causes.
If the dog gets a clean bill of health, the owner should then seek help from an animal behaviour specialist, preferably one that is a member of the Animal Training and Behaviour Council.
But she acknowledged that the cost of living crisis meant that some owners may not be able to afford the help they need.
MPs on the Commons committee are investigating pet welfare and abuse in the wake of the pandemic, which saw an explosion in dog ownership.
The UK's dog population increased from an estimated 10 million to 11 million over two years, an earlier hearing was told.
Much of that was driven by demand for fashionable breeds such as French bulldogs and cross-breeds such as Cockapoos, with more animals being imported without pet passports or bought from the internet.
On Tuesday, Pip Griffin, a senior warden in the Worcestershire area, told the committee dog pounds were at bursting point with abandoned animals, meaning imports should not be needed.
The committee also heard concerns about canine fertility clinics, which have sprung up to meet demand for "designer" breeds, but which have been accused of carrying out blood tests, artificial insemination and even caesarean sections without trained veterinary surgeons.
The committee is also looking into the welfare of cats, rabbits and other domestic pets.