As the summer holidays approach, a consumer group has warned that a 'postcode lottery' on foreign currency exchange rates may be leaving some holidaymakers short-changed.
Which? sent mystery shoppers to different currency exchange providers across Britain and found there was a significant regional variation in the amount of euros it was offered in exchange for £500, even between branches of the same company, including the nation's biggest high-street supplier, the Post Office.
The researchers were offered 578.50 euros in branches in London and Glasgow, but only 565 euros in Birmingham, Haverfordwest and Sheffield.
Looking across 10 UK locations on March 28, Which? said Thomas Cook offered the best in-store deal of 585 euros in Sheffield, although the difference from its lowest offer of 553.80 euros in Manchester was worth just over £25 - potentially enough to pay for a taxi to or from the airport.
But using an eight-week average, the Post Office and Thomas Cook offered the best rates, despite having the biggest regional variations.
Bob Atkinson, of Travelsupermarket, said the differences were not necessarily due to the firms "taking advantage".
"It's really in local situations where you have a slightly different mix on any high street of who is going to offer the best possible rate," he told Sky News.
"So if somebody comes in and offers an incredible deal, someone else is going to go in and match it."
Which? executive director Richard Lloyd said: "While we understand that local competition may play a part in the rates set, it is unfair that people are missing out on the best currency exchange rates just because of where they live.
"Holidays don't come cheap and these anomalies don't help hard-pressed consumers. Currency providers need to make sure their rates are consistent to ensure a fair deal for everyone."
However, the study found that Marks & Spencer offered the same rates nationwide and Sainsbury's showed no significant regional variation. Although there was some variation between branches of Asda and Tesco, this was not common, the watchdog said.
In an extended study, Which? found notable differences in the rates offered by online and central London foreign currency providers over an eight-week period, although it warned that buying in branch was more secure.
"Unlike other financial transactions, currency exchange is not regulated by the Financial Services Authority or covered by the Financial Services Compensation Scheme.
"This means that if a firm goes bust before supplying your order as Crown Currency Exchange did in 2010 your money may be at risk," it said.