Most retired households receive the bulk of their retirement income from private pensions and annuities, according to the Office for National Statistics.
Until 1999 to 2000, the ONS found the state pension was consistently the largest source of income for retired households but this has changed over the last 16 years.
This is despite the fact that after allowing for inflation the state pension doubled from an average of £4,700 in 1977 to £9,000 in 2014 to 2015.
In its report, called Household disposable income and inequality, financial year ending 2015, the ONS said the median income of retired households was largely unaffected by the economic downturn and rose by 7.7 per cent - or £1,500 - between 2007 to 2008 and 2014 2015.
It said: “Overall, the proportion coming from cash benefits (including state pension) has fallen significantly, from 64.7 per cent in 1977 to the current level of 47.1 per cent.
“This has mainly been due to growth in the percentage of retired households receiving income from private pensions and annuities, which rose from 44.5 per cent in 1977 to 79.6 per cent in 2014 to 2015.”
In 1977, the average income received by retired households from private pensions was £1,600, accounting for 18 per cent of the gross income of this group.
By 2014 to 2015, retired households received on average £10,300 from private pensions or annuities, equivalent to 43 per cent of their gross income.
Meanwhile in 2014 to 2015 investments and other income made up less than 10 per cent of retirees income.
The report added: “Retired households are much more likely to be towards the bottom of the overall income distribution than at the top of the distribution.
“Whereas retired households made up 34.4 per cent and 39.4 per cent of the bottom and second quintile groups respectively, they only made up 12.6 per cent of the richest fifth of households in 2014 to 2015.
“However, the relative position of retired households has improved over time. For example, 10 years previously, retired households made up 45.7 per cent of the bottom fifth and 6.1 per cent of the top fifth of the income distribution.”
