Many of tomorrow's pensioners will become self-employed consultants, online traders or run odd-job businesses to make ends meet in retirement, according to research.
A generation of "Wearies" - Working, Entrepreneurial and Active Retirees - could be forced to work into their 70s and beyond due to the looming pensions crisis.
It will result in the traditional image of the pensioner relaxing in old age changing completely as many will simply not be able to afford to retire, experts say.
Effectively ruled out of employment by age, they will instead set up their own businesses, according to the study for Friends Life by think-tank Future Foundation.
More than half (51%) of those already retired said they would be prepared to do part-time work to boost their pensions, the research found.
But the figure jumped to three-quarters (75%) among those who are yet to retire.
Of those still working, 89% said they would be prepared to work part-time, run firms, start trading online or live with others to supplement their incomes.
Martin Palmer, head of corporate benefits marketing at Friends Life, said: "We're expecting the traditional image of the pensioner with slippers and rocking chair to change completely.
"Many will not have saved adequately for a secure retirement and, with years of fiscal austerity taking their toll, by 2020 many people in their 70s simply will not be able to afford to give up working.
"Necessity is the mother of invention and 'Wearies' will be among the most innovative and entrepreneurial contributors to the UK economy, despite their senior years."
A total of 59% said they would run "a small, one-person business from home" and 21% would consider gardening for elderly neighbours or for the local council.
One third (33%) said they would rent out a spare room to a lodger, and 14% said they would think about moving in with other family members.