By Bob Howard
BBC Radio 4's Money Box
Don McDonald claims notes he was given by the Post Office were fake
Some holidaymakers are claiming foreign currency they have bought through UK bureaux de change have included forged notes.
They told the BBC the fakes were discovered when they tried to use them in shops and hotels abroad.
A leading lawyer has called for greater consumer protection for these types of cases.
However, UK foreign currency providers deny they are the source and insist their security systems are secure.
Don Macdonald from London ordered $1,100 (£690) from the Post Office's online currency service in April to go on holiday to South America.
After some of his notes were rejected, he discovered 32 of the $20 bills he was carrying were forged, amounting to $640 (£400).
He believes his only source of US dollars was the Post Office and there had been no opportunity for anyone to switch genuine notes for forged ones whilst he had them in his possession.
When he returned home, he sent the counterfeit US dollars to the Post Office.
It confirmed the notes were forged but refused to refund him.
Mr Macdonald told BBC Radio 4's Money Box he will pursue his claim for compensation.
"They've got all the dollars, they accept they're counterfeit. That suggests to me, there are a lot of serious flaws in their systems," he said.
"They haven't suggested any way this could have occurred."
The Post Office said it has investigated but has found no breach of its security.
It insisted: "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."
Gail Chandler from Kent also said she was sold counterfeit US dollars for a holiday to Las Vegas.
She was detained by the US Secret Service after they discovered eight of the new $100 bills she had changed were counterfeit.
"They said stand up please, put your hands behind your back, they are counterfeit notes and they marched me off," she told the programme.
She was released after she was able to prove she was a regular visitor to Las Vegas and that she had a receipt from Thomson, her foreign exchange provider.
Thomson has denied liability because it insists all the dollars it had given Ms Chandler were used notes.
It says if the fake dollars were new, as Ms Chandler says, its bureau could not have been the source.
"Our US currency undergoes rigorous checks for forgery before being sold on to customers," it said.
"Our staff are trained to spot counterfeit notes and systems are in place to ensure any forgeries are intercepted."
If a company does not offer compensation, consumer law expert Ingrid Gubbay said people's legal options are very limited.
"Counterfeit goods are normally covered by the Sale of Goods Act. Currency falls outside of that," she told the programme.
"This really highlights the need for some level of consumer protection."
BBC Radio 4's Money Box was broadcast on Saturday 23 May 2009 at 1204 BST. Download the free