All cats in England must be microchipped and registered on a database from Monday under new laws.
The new legislation applies to all cats aged 20 weeks and older, including indoor cats.
The RSPCA said that one in 10 cats going into their centres were still not microchipped, despite the incoming law change.
Pet owners found breaking the rules face a £500 fine if they do not comply within 21 days.
Data from a Cats Protection survey suggested about 1.9 million cats in England were not microchipped. The charity said owners of about 300,000 cats were not sure if they were microchipped or not.
Microchipping involves a small chip being “quickly and simply” inserted under the animal's skin to give the pet its own unique code.
The chip can then be scanned and matched to the owner's contact details that are kept on a database.
Microchipping cats is not yet a legal requirement in Scotland, Wales or Northern Ireland.
Vet Rory Cowlam, RSPCA ambassador and CBBC presenter, said: “The thought of a pet going missing and never being able to find them is a pet owner's nightmare, which is why microchipping is so important.
“Having microchipped many animals myself, I can assure you it’s quick, easy and is the best way to ensure that if your pet does go missing, or becomes injured, they can be reunited with you. “
The animal charity said the new rules would improve cat welfare, help track down missing pets and could stop owners abandoning their animals.
The RSPCA said last year more than 7,500 abandoned cats were reported to the charity and more than 1,500 so far this year.
Microchipping can be done by a vet or local rescue and rehoming centre and usually costs between £20 or £30. It is also the owner’s responsibility to update the database if they move home or their details change.
Cats Protection said recent research it had carried out showed cat owners who had not microchipped their pet felt it was unnecessary because their pet either did not go out or did not wander far.
Madison Rogers, from the charity, said: “Some owners think they are never going to go through the trauma of losing their pet cat, but in the last year 115,000 pet cats in England went missing and never returned home so this is much more common than people think.”
