Ports and airports have been told to be on the look-out for people trying to enter the UK on fake visas following the theft of official documents.
The stolen documents all carry serial numbers
Some 900 vignettes - the blank stickers which house the visa stamp - were taken as they were transported to the Foreign Office in London from a printing plant.
There are indications the papers may have been sold in India and Pakistan.
A number of people from South Asia have been stopped at Heathrow with false visas in their passports.
The vignettes each carry a serial number and the counterfeit visas could easily be identified, the Foreign Office said.
Police are investigating the theft, which took place in March.
A Foreign Office spokeswoman said it was unclear how the vignettes were taken.
She said the theft was discovered immediately and all UK ports and airports were informed within two days.
"We are confident we have sufficient measures in place to stop people from entering the UK on the fake visas," she said.
"Security is very stringent."