A lawyer believes consumers should be offered greater legal protection after holidaymakers have claimed money they bought through UK bureaux de change included forged notes.
Don MacDonald told the BBC that he found that currency he purchased from the Post Office was fake when he later attempted to use it in shops and hotels abroad.
The London-based holidaymaker ordered $1,100 (£690) from the Post Office's online currency service in April before his trip to South America.
However, after some notes were rejected, he discovered that $640 of the money was forged.
When Mr McDonald returned home he sent the counterfeit US dollars to the Post Office, but they refused to offer him a refund.
A Post Office spokesperson said: "The Post Office only supplies mint condition notes for these orders, supplied directly from the US Federal Reserve Bank through our currency partner, First Rate. We are very confident that the currency we issued was genuine."