Nearly a fifth (19 per cent) of employees secretly juggle two competing jobs, a practice termed "polygamous working", with almost a quarter (24 per cent) believing it "justifiable".
The findings come from a survey for fraud prevention service Cifas.
The research, polling 2,000 employees across sectors like HR, engineering, finance, and IT, also found 19 per cent of individuals, or someone they knew, used bogus references to cover employment gaps.
Conducted by Opinion Matters in July, the survey focused on staff in UK companies employing at least 1,000 people.
Mike Haley, CEO of Cifas, said: "Our Workplace Fraud Trends research doesn't solely reflect individual choices. It reveals systemic blind spots to a whole range of rising threats impacting the workplace - from polygamous working to UK professionals using fraudulent reference houses."
Keith Rosser, chairman of the Better Hiring Institute and director of Reed Screening, said: "Hiring fraud, driven by modern working styles and AI, is a quickly growing challenge for organisations."
Cifas is encouraging employers to scrutinise people's qualifications and work history thoroughly.
The figures follow new research revealing three in five workers in low-paid jobs have skipped meals, cannot heat their homes or have taken out a payday loan to cover essential supplies.
The Living Wage Foundation said its survey of 2,000 workers showed the "devastating impact" of low pay on people's lives.
Two in five said they have been forced to use food banks, rising to more than half of low-paid workers with dependent children.
The research was based on a survey of workers paid below the voluntary so-called real living wage of £12.60 an hour and £13.85 in London.
