Airports' bureaux de short change

It found that traveller buying 500 euros could pay £35 more at the airport than on the High Street.

Which? paid £351 at a Marks & Spencer bureau de change, which offered a rate of 1.424 euros to the pound and did not charge a commission.

But the airport exchange it was offered a rate of 1.323 - and was charged 2 per cent commission.

Which? said: "We checked the exchange rate when buying 500 euros in both cash and travellers' cheques at ten different providers on the same day.

"The banks and high street providers, including M&S and the Post Office, were offering similar deals.

"But the bureaux de change at airports were much more expensive." The calculations were based on transactions made in March.

Although the rates will have changed since then, the difference could be similar.

Which? advised those buying foreign currency to check if their bank offers customers preferential rates.

Ordering online, for example from Travelex, will often provide a much better deal, it said.

The consumer group also issued a warning over the cost of using plastic cards overseas, as the majority carry hidden transaction fees.

Using a debit card is generally a better option than a credit card for taking money out of a foreign cash machine, it said.

Debit card withdrawals generally attract a handling fee of 1.5 per cent, while credit cards often demand 2 per cent - and there are also extra flat fees to be taken into account.

This means that taking the equivalent of £100 out of a cash machine overseas in euros or dollars, using a debit card, can cost up to £4.50.

The same transaction with a credit card could be up to £5.75.

However, it is often cheaper to use a credit card to make purchases. The handling fee for purchases on a credit card is generally 2.75 per cent.

With debit cards it is often the same - but there is another flat fee of up to £1.50 per transaction.

Which? said some card providers offer better deals than others.

Nationwide's FlexAccount Visa is the best debit card deal. There are no fees associated with purchases or cash withdrawals.

For credit cards that can be used anywhere, the Post Office Platinum MasterCard and Nationwide's Gold Visa are the cheapest. Neither imposes a fee on purchases.

The Saga Visa has a charge of 1 per cent on use outside of Europe and the Liverpool Victoria Visa has a charge of 2.75 per cent outside Europe.

Editor of Which? Money, Martyn Hocking, said: "Don't leave changing your holiday money until the last minute.

"Check if your own bank offers preferential rates, and shop around to get the best deal on the high street or online."

The consumer group has found that banks have taken to blocking cards which suddenly show a number of overseas transactions as a security measure.

It says travellers would be wise to tell their bank they are going away to avoid such a problem.