People in Northern Ireland have around £140 in leftover holiday money sitting at home on average, according to estimates.
With holidays cancelled or postponed over the past year amid the coronavirus pandemic, it is thought around 23m adults have spare foreign cash.
Men have more stashed away at home than women, at an average of £188 compared with £117.
Post Office Travel Money commissioned a survey of more than 2,000 people across the UK in February, which found that more than two in five (42%) people have leftover holiday currency. Based on UK adult population figures, it estimates that 22.72m are holding leftover cash, with the Euro and US dollar accounting for much of the spare money.
The total holiday currency "cash stash" could be as high as £3.5bn, based on the average amount of leftover foreign currency at just over £150, the research found.
People in Scotland were found to have the most holiday cash tucked away on average, at around £263.
In Northern Ireland that figure is £141, while in Wales it is estimated at £177.
The English regions ranged from the South West (£231) to the East of England (£92).
Some 72% of those who had leftover currency said they were keeping it to use on a future holiday, suggesting strong pent-up demand for trips abroad.
Nick Boden, head of Post Office Travel Money said: "Our research shows that there are billions of pounds worth of foreign cash hidden away at home so now might be the time to check how much you have.
"If it turns out to be currency for Australia, New Zealand, Norway or Sweden and you are not planning to travel to these countries in the foreseeable future, now might be the time to change it back into sterling.
"These have risen in value against sterling, and you will get more cash back.
"Equally, if you are planning a holiday abroad when the rules allow it, it is worth considering destinations where sterling has risen most in value.
"The pound is worth over 7% more against the euro than a year ago, but other currencies have weakened more.
"The Turkish lira is down almost 35% and Caribbean currencies have weakened by 18% to 29%."
The percentage of people who have leftover holiday currency