Around 34 per cent of people admit they have helped themselves to change from their partner's wallet or purse, but three-quarters of these thought it was acceptable, according to the discount voucher website MyVoucherCodes.co.uk.
Just over half of those surveyed said what belonged to their partner also belonged to them, while 23 per cent said their partner would not mind and 11 per cent thought their partner would do the same.
The research, based on responses from 2,748 people, also found there were regional variations in how much people thought it was acceptable to take.
Those in south-east England said they would happily take an average of 10.20 without feeling the need to tell their partner, but those in north-west England would not take more than 1.50 on the sly.
Farhad Farhadi, personal finance expert at MyVoucherCodes.co.uk, said: "In a relationship, finances are often shared, especially in the case of a joint account where two incomes merge into one.
"With that said, the poll shows it's now not uncommon for people to take money from one another and later forget to mention it."