The firm, a sponsor of the Institute of Financial Planning, found the average amount kept secret was £2,004. The most common reason for keeping the cash secret was for financial security in the event of a break-up or to buy items for themselves.
Some 6.2m people had secret debts and even secret mortgages, averaging at £9,546. These were used for covering everyday living costs and paying off other outstanding debts such as credit card bills.
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Some 14 per cent of people admitted their partner did not know how much they earnt as they never discussed earnings or they had misled their partner into thinking they earn more or less than they actually do.
Vince Smith-Hughes, retirement expert at Prudential, said: "Having conversations about finance may take some couples out of their comfort zone but keeping money hidden from a partner could be a real barrier to long-term security.
"Saving money, whether for a rainy day or a comfortable retirement, is imperative but doing it openly and honestly is the only way to ensure that savings are maximized between both partners."
The survey questioned 2,000 people co-habiting with a partner on their attitudes to Financial Planning for couples.

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