Researchers found 48 per cent expect the basic state pension of up to £110.15 per week to be enough.
Some 53 per cent of men rely or plan to rely on their basic state pension compared to 44 per cent of women, according to the survey by Nationwide.
Only 39 per cent of those aged between 25 and 34 plan to use the basic state pension to fund post-retirement leisure activities, but this rises to 58 per cent of people aged over 55.
Rob Angus, Nationwide's head of protection and investments, said: "Our research suggests a misguided view that the basic state pension will be sufficient to fund life in retirement.
"If that is their only source of income, retired people are unlikely to have enough to achieve the lifestyle that they hope for."
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Some 22 per cent plan to continue working part-time during retirement in order to fund any leisure activities.
ISAs, inheritance, private pensions and property are other forms of funds that those surveyed said they would rely on.
Mr Angus said planning and saving for retirement is vital.
He said: "In fact, it is one of the most important financial steps people will ever make, which is why it is concerning that around one quarter of UK adults have not started planning for retirement.
"It is certainly something that can never be started too early.
"The longer it is left, the more disastrous the outcome for people's finances and the tougher those older years are likely to become."