Stolen old-style bank notes worth several thousand pounds could be circulating in Berkshire, police fear.
Detectives are now warning banks and businesses to be vigilant after a batch of £20 notes, thought to be worth £5,000, were stolen from a house in Windsor.
The burglary happened on Thursday, 22 January between 1900 GMT and 2030 GMT.
Officers investigating the incident want any banks which have been asked to exchange a large sum of old-style £20 notes over the past few days to contact them.
They also want to hear from businesses which may have had to turn away customers wishing to use them to settle a transaction.
A police spokesman said: "The notes are not legal tender, but they can be still changed at banks."
At this stage, it is unclear exactly what series the old notes originate from.