Nearly a quarter of couples surveyed, all aged over 40, were concealing debts, Prudential found. Nearly half said these had arisen from borrowing to cover every day living costs, while a third overspent due to an emotional event such as the break up of a previous relationship.
Some 16 per cent had borrowed money to make payments to an ex-partner, for example for child support, while 26 per cent admitted to keeping a stash of money secret from their partner, with an average secret savings pot of £4,000.
A fifth said their hidden money was to pay for something specific in future such as a car or a dream holiday, while 22 per cent are keeping it to help fund their retirement.
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Vince Smith-Hughes, retirement expert at Prudential, said: "Many people may have perfectly valid reasons for keeping a stash of savings or personal debts secret from their other halves. "However, these financial secrets could pose a serious risk to a couple's future retirement income.
"For example, if a couple reach retirement with savings that are secret from one another they may have missed out on years of tax relief that they would have been entitled to if the money had been invested in a pension.
"Meanwhile, making repayments in retirement on an unexpected debt will have an obviously negative impact on a couple's income."
Furthermore, some respondents were also found have never told their partner what they earn – with 13 per cent making this admission.
One in ten claim their partner has a false impression of their earnings, with 38 per cent of those who do not tell the whole truth about their income saying their basic salary is higher than their partner thinks it is.
The study found 31 per cent have received at least one bonus payment that they kept quiet from their partner and 31 per cent admitted not discloing additional income earned through overtime or 'cash in hand' work.
Mr Smith-Hughes said: "Having those potentially awkward conversations so that both partners get a full understanding of their joint financial circumstances is an important first step for a couple planning for a comfortable retirement."
As to why they keep the information secret, 51 per cent said the main motive was to maintain independence and 22 per cent said they do not trust their partners' financial decision making.One in six keep secret funds to protect themselves financially in case of a future split – but 29 per cent do so to buy gifts for their other half.