Sterling is now at its lowest point of the year against the US dollar, according to Post Office Travel Money.
UK tourists visiting America are now getting 7.2% fewer dollars to the pound than a year ago and 29% fewer than when the pound was worth 2.0 dollars in 2007.
Post Office Travel Money head Andrew Brown said: "Just a month ago sterling was on the rise against the dollar but it has fallen back steadily ever since and is now worth 6% less than in early May.
"You would have to go back to the beginning of the year to see such a low rate and if the pound falls further, it will be at its lowest value against the dollar for two years."
He went on: "Obviously, this is disappointing for families planning trips to the United States. However, our holiday costs barometer reveals that people travelling to the US will still pay slightly less than in the UK for drinks like coffee and Coca-Cola and significantly less when shopping for designer clothes, toys and other retail items."