If the worst happened more than a third would turn to partners, parents or even be forced to sell their house, the study found.
Nearly a quarter favoured relying on their partner (23%) while 7% would be forced to sell their home and 4% would call on their parents for financial assistance.
Just 15% have insurance protecting their income against the risk of illness or a serious accident with only 7% receiving it as part of their employee benefits package, MetLife found.
Men are the least prepared with 45% admitting to having no Plan B compared to 38% of women.
But women are twice as likely to rely on their partner for financial help with 30% happy to rely on their partner compared with 15% of men.
Women are also three times (6% vs 2%) more likely to turn to their parents for a financial boost.
Workers in Southern England are the most unprepared with 44% admitting to having no Plan B falling to 34% for London – the lowest regional figure.
In terms of being forced to sell their property, Scotland was double the national average at 14% and also topped the table in terms of the region most likely to rely on parental assistance (6%).
Stephanie Baillie, employee benefits director, MetLife UK said: “Most of us understandably do not like to think of the risks of not being able to work as a result of ill health or a serious accident – but for such a high proportion not to have any financial back up plan is worrying.
“Organising your finances and giving yourself a safety net is crucial. While no one wishes to consider losing their job or being unable to work, factoring in insurance to protect your income should be as much part of financial planning as saving for retirement or for a deposit for a house. ”