The survey of over 2,000 jobseekers, all currently employed, found that over a quarter (28%) spend more than three hours per week looking for jobs at work, 16% more than five hours and 7% more than ten hours.
If this were consistent across all UK jobseekers, it would amount to a weekly total of over 14 million hours, costing employers more than £250 million every week.
This time is not just spent surfing the internet either. While 60% of respondents say they do search online for jobs at work, half also update their CV, 49% apply for roles and almost a quarter (23%) carry out telephone interviews from the office.
But it appears all this job-hunting hasn't gone unnoticed, with 40% of respondents saying they think their boss knows they are looking and 12% saying they have actually been caught by their boss or another colleague. One in 50 of those questioned has actually lost their job this way.
Furthermore, over a third (39%) of respondents have attended job interviews during working hours, with a medical appointment being the most popular excuse for absence (23%), closely followed by a home delivery (22%) and a pet emergency (14%).
Worryingly, 6% of respondents fabricate the death of a relative to explain their absence, more than would fake sickness (1%) or a domestic emergency (5%).
Male respondents are the biggest culprits when it comes to job-hunting at work, with 30% spending more than three hours per week looking for a job, compared to a quarter of women surveyed.
Regionally, those surveyed in Anglia spend the most time job-hunting at work, followed by London and then the South East.
Age-wise, youngsters are the biggest culprits, with 21% of 18-to-30-year-old respondents spending more than five working hours looking for a job. Surprisingly, those in the 51-to-60 age bracket are also serious offenders, with only slightly fewer (20%) doing the same.
Isabelle Ratinaud, spokeswoman for Monster UK & Ireland, said: "Looking for a new role can be time-consuming and many people are clearly so desperate to move that they will spend hours looking and applying for roles while they are supposed to be doing their current job.
"But while this may be tempting, it is important that jobseekers try to limit their hunt to lunchtimes, evenings and weekends, wherever possible. If your boss finds out you're job-hunting on their watch, this could not only affect your reference, it could even result in disciplinary action or dismissal - which could make finding a new job even harder.
"To reduce the chance of being found out, jobseekers should make use of all the new technology available: iPhones, iPads and other mobile devices mean you can now search and apply for roles from pretty much anywhere, at any time of the day, whether it's on the way to work or in a coffee shop round the corner - but during an authorised break of course."
