Only one quarter of workers around the world have permanent jobs, according to a report by the International Labour Organization (ILO).
The remaining three quarters of the workforce are employed on temporary or short-term contracts, along with informal jobs often without a contract.
The ILO also found that many workers not in full-time employment have no pensions or benefits.
The study covered about 84% of the global workforce, said the UN agency.
Part-time jobs outpaced full-time ones between 2009 and 2013 in a majority of countries where the data was available.
The ILO says flexibility in employment does have some advantages, but it also adds to the risk that workers will be exploited.
The study shows an increasingly diversified global workforce, said director-general Guy Ryder, with some forms of "non-standard" work helping people get a foothold into the job market.
"But these emerging trends are also a reflection of the widespread insecurity that's affecting many workers worldwide today," he added.
Women were a big part of the current trend of rising part-time employment, according to the ILO.
They accounted for 24% of people working less than 30 hours per week across 86 countries - nearly double the percentage of men at 12.4%.
Meanwhile, the income gap between permanent and non-permanent workers has also increased.
Benefits such as pensions and unemployment benefits are still mainly available for permanent employees, the study found.
The ILO is calling for policies by governments to ensure income security for all types of workers, not just those on "stable contracts".
"The key issue is to match regulation to an increasingly diversified labour market," said Raymond Torres, director of the ILO research department. "Well-designed regulations can support both economic growth and social cohesion."