THE weakness of sterling against other currencies – especially the euro and dollar – means millions of British travellers face a double whammy on foreign spending this year.

The poor conversion rate is an obvious trap, but most people find this out if they change cash before they go on holiday.

It’s a painful lesson if you have to settle for the conversion rate at UK airports.

To add insult to injury, however, the banks have sharply increased fees and charges for using credit and debit cards abroad – and many travellers will realise their pockets have been picked for a second time only when card statements arrive after their holiday.

Research from uSwitch, the independent price comparison and switching service, predicts British holidaymakers will pay £754m in card charges on overseas purchases – £401m on debit cards, and £353m on credit cards – on “plastic” spending topping £22bn this year.

Andrew Hagger at Money net.co.uk says the obvious trap is debit cards, which are free to use in the UK.

“Abroad, however, purchases are subject to a conversion fee (2.75 per cent to 2.99 per cent) plus a charge of up to £1.50 per transaction, so there is a strong argument for not using debit cards on many small transactions.

“Plan your spending. If you are in a villa, don’t go to the supermarket each day for beers, when you could buy a week’s supply in one transaction.

That keeps charges down.

“Similarly, the case for fixing car hire before you leave the UK is probably stronger in 2009. You know exactly what you will pay, and can’t be caught by dips in the exchange rate. And you won’t sign a form in an airport queue, which might incur extra charges.”

Mr Hagger says some families could get hit by charges of £100-plus on their use of cards for purchases and cash withdrawals.

On some transactions, total charges approach six per cent.

uSwitch says 17 banks now impose transaction fees on debit card purchases abroad – against nine last year. Tour operator Thomas Cook, which launched its credit card with a flourish last summer, imposes a foreign exchange fee of 2.75 per cent from Saturday.

On most cash withdrawals by cards, there’s a conversion fee (2.75 per cent to 2.99 per cent) plus a withdrawal charge of £1.50 to £5.

Michelle Slade at Money facts.co.uk say most credit cards charge a foreign usage charge, some as much as 2.99 per cent, for both purchases and cash transactions, each time a card is used.

“Taking cash out on a credit card is never advisable, even in the UK. The interest rate is much higher, and you are charged interest from day one.

“Abroad, you will be charged a cash handling feeplus a foreign usage charge.

Take out £100 on an MBNA Europe Bank credit card, for example, and the charges are £5.99.

“With Nationwide BS and Thomas Cook Financial Services adding foreign usage charges soon, only the Post Office will make no additional charges for using a card abroad.”

Peter Harrison, Money supermarket.com travel money expert, also likes the Post Office Platinum credit card.

“Unlike bigger name rivals, it offers free purchase protection, zero per cent on purchases for three months and no overseas charges,” he says.

“It’s a better bet than other cards charging up to three per cent for purchases abroad.

“Nationwide’s FlexAccount has the cheapest debit card for overseas cash withdrawals – levying a fee of one per cent or less. In contrast, a £100 withdrawal on other debit cards can incur a £2 transaction charge and three per cent foreign exchange fee.

“If you are stuck with uncompetitive debit or credit cards and haven’t time to get a new one, FairFX and Caxton FX prepaid foreign currency cards are a good alternative.

“Because they are prepaid, they offer no credit facility, are approved instantly and – to avoid overspending – only let you spend what is on the card.”

Moneysupermarket.com’s table of the top six cards, providing the cheapest way of spending either 500 euros or 500 dollars, is headed by the Post Office Credit Card, followed by Nationwide BS Flex- Account Debit Card; Thomas Cook Credit Card, Nationwide Gold Credit Card, Caxton FX and Fair FX, (both prepaid cards).

Mr Harrison says finding the right payment vehicles for a foreign holiday is more important as sterling weakens.

He goes abroad with both Nationwide BS debit and credit cards – the latter for consumer protection, although others see it as a way of spending money they haven’t yet earned – which are available to Nationwide account holders.