Britons have over £1 billion worth of foreign currency sitting unused at home, new figures reveal.
This is because four out of five holidaymakers bring money back with them from abroad, money that is rarely converted back into sterling, research for M&S Money reveals.
"This year, 14 million travellers brought home at least £50 from their trip abroad," David Mackay, commercial director at M&S Money commented.
Overall, more than one traveller in four (28 per cent) arrives back in Britain with £50 worth of dollars, dinars, or euros.
"If they changed it back rather than forgetting about it, they could put the money towards a deposit on their next holiday or a short break," Mr Mackay added.
This hoarding mentality has become more common since the adoption of the single European currency - with holidaymakers believing they are likely to be able to use their euros again in the near future.
But a large number of Brits are returning from far flung locations which they are less likely to return to.
"Holding on to euros is understandable, but the 42 per cent who holiday outside the euro zone would be better off changing it back to sterling," Mr Mackay noted.
M&S research reveals that the average holidaymaker returns home with £16 worth of currency, with three out of four travellers not bothering or forgetting to change the money back into sterling.
A third of these leave it burning a hole in a drawer, wallet or handbag.
But for the majority, this inactivity is seen as being sensible with their money, with six people in ten saying they will definitely use their foreign currency in future.