Customers face a postcode lottery when buying currency in their local Post Office, it has been claimed.

Research carried out by travel money firm FairFX and shown exclusively to the Press Association found that rates varied significantly at certain Post Office branches.

It found that someone buying £500 worth of euros yesterday could have been almost 50 euros worse off depending on which branch they walked in to.

Darren Kilner, currency expert at online FairFX, said the research highlighted "huge inconsistencies" in the rates customers were being charged in branch.

"Depending on where you live or what Post Office you visit, the exchange rate varies considerably," he said.

"Our mystery shoppers found that if you were changing £500 in Ashford, Middlesex, you'd get 47 Euros less than if you were changing the same amount, at the same time, on the same day in Durham. This inconsistency is not transparent.

"The Post Office needs to treat its customers more fairly and have one consistent rate across the board and across the UK for its customers."

FairFX visited 20 branches on Thursday morning to capture rates on the currency boards out front.

The Post Office has disputed the findings. It claimed one of the lowest rates spotted by FairFX was a result of incorrect data being displayed in the window of one of its branches.

It added that it was currently offering more generous rates in certain branches as part of a four-week trial, making the gap between best and worst far larger than it would normally be. Normally there would not be difference of more than 20 euros on a typical £500 purchase.

Andrew Brown, head of travel money at the Post Office, rejected claims that customers faced a postcode lottery.

"The vast majority of the Post Office's branches - 11,100 out of 11,500 - offer the same exchange rates on all currencies," he said. This works out to 96.53% of its network.

"However, over the past three years, aggressive pricing tactics by foreign exchange competitors in some locations has meant that we have had to respond to this common retailing practice by making some small adjustments to the rates offered in around 400 of our branches.

"Post Office's online rate is consistent around the country and customers can have cash delivered free to branch or home.

"We also incentivise customers with great offers, available across the whole network, which include better rates on transactions of over £500 and £1,000 and weekly special deals on specific currencies."

Almost nine in 10 (87%) Post Office travel money customers choose to buy in branch rather than online.

The Post Office added that it was not unusual for forex retailers to have different rate in line with competition.