Six per cent said they had regular jobs and worked illegally part-time, while five per cent said illegal work was their only source of income.
The agency said the survey did not reflect the real situation about the scope of illegal labour because some of those asked had reservations, saying if they were to say the truth it might be misused. GfK said the actual percentage of people working illegally was certainly higher.
The survey was conducted in May. Fifty-eight per cent of those who said they worked illegally worked in construction, 13 per cent in catering, 12 per cent in tourism and three per cent in the agriculture.