UK shoppers may have lost a total of nearly £100m in unused vouchers during the coronavirus lockdown, the consumer group Which? has estimated.
A survey of 2,000 adults suggested a quarter of people possessed a voucher that expired during lockdown.
Many retailers contacted customers and offered to extend them - but a third of those with an expiring voucher did not get an extension and lost the money.
Which? urged people who were unable to use their vouchers to contact the shop.
It also warned of the risks of buying new vouchers because some well-known retailers had recently collapsed - and added that further coronavirus restrictions could make it difficult to spend them.
Non-essential shops and businesses closed in March for several months, as part of the government's lockdown restrictions to deal with the pandemic.
Many shops automatically extended the time that customers could spend their vouchers - as well as extending their return periods.
Shops eventually reopened throughout June (in Northern Ireland first, followed by England, Wales and then Scotland) with some long queues outside stores.
'Contact the company'
In its survey carried out in August, Which? found that nearly half of people with an expiring voucher had it automatically extended.
One in seven - or 15% - of people with a voucher had to to request an extension, with one customer telling Which? that a company told them "hard luck, basically".
But 36% - which could equate to an estimated 3.1 million people, according to Which? - did not receive an extension on their shopping vouchers.
Which? says the unspent vouchers were worth £31.70 on average, meaning around £98m was likely to have been lost across the whole of the UK.
The consumer group advised anyone who has a voucher that expired during lockdown to contact the retailer.
"If you have a voucher you were unable to use it is worth contacting the company," said consumer rights expert Adam French.
"Anyone considering buying a voucher should be aware of the risks, as some well-known retailers have collapsed in recent months and further coronavirus restrictions could make it difficult to spend vouchers and gift cards."