Popular foreign-exchange providers like banks, the Post Office and airport bureaux de change have been increasing profit margins just as travellers suffer some of the worst exchange rates on record.

Many currency providers, including the Post Office and Marks & Spencer, boast they do not charge commission, but they make their money by charging a margin on top of the wholesale exchange rate.

Currency broker Fair FX found the Post Office, where nearly a third of travellers obtain their euros and US dollars, has almost doubled its margin in the past three months from around 2.9% to 5.6%, even though it charges no commission.

That means its mark-up on exchanging £1,000 three months ago has increased from £31 to £56, according to Fair FX.

Lloyds TSB’s in-branch bureau de change has increased margins from about 3.4% to 7%, while Marks & Spencer has raised its mark-up from 3.34% to 5.29%.

The analysis comes as consumers holidaying in Europe this ski season face a big increase in costs because of sterling’s slide. Over the past 12 months, holidaymakers’ spending power in the eurozone has fallen about 24% while travellers to America have seen their pound buy 26% less.

However, sterling has rallied so far this year as traders bet that Europe’s woes could be even worse than our own. Marc Cogliatti at the foreign-exchange specialist HiFX said: “The transition into a new year has opened up the debate as to whether the pound is now too cheap or whether we will see further weakness in 2009. For the time being the market seems to agree that sterling is undervalued.”

Despite this, holidaymakers are still being squeezed. It has emerged, for example, that some credit-card providers increased foreign loading fees — charged when a debit or credit card is used abroad — just before the Christmas holidays. Fees on the Citi, Shell, Easyjet and American Airlines credit cards increased from 2.75% to 2.95% at the start of last month, according to analysts at Defaqto. The change affects new and existing customers.

HSBC increased its foreign loading fee from 2.75% to 2.99% for new customers from December 23. Existing customers will have until March before fees rise. Most currency providers set rates at a point in the day so have to cover themselves for daily interbank-rate fluctuations.

The Post Office would not reveal the exact time it pegs its rates to the interbank rate, saying the information was “commercially sensitive”. It argues it is therefore impossible for Fair FX to work out its exact margin. It said: “As the UK’s largest travel-money provider, the Post Office reviews the exchange rates that it offers customers daily, to ensure we can continue to provide them with a highly competitive deal.”

M&S, Travelex and Lloyds also said their margins fluctuated daily and disputed Fair FX’s findings, although they declined to provide details of their mark-ups.

Here we offer some tips to get the best deal on your holiday cash.