More than 300,000 workers could call in sick on Monday on what has become the worst day in the year for office absenteeism, research suggests.
'Duvet days' are popular this time of year, it is claimed
A mixture of cold weather, financial problems and post-Christmas withdrawal symptoms has made the first Monday in February the most traumatic for staff.
According to Employment Law Advisory Services (ELAS), "fake" absenteeism could cost the economy £27m.
But unions said it was "insulting" to suggest people were taking "sickies".
Many people were likely to take a "duvet day" on Monday in the face of low temperatures and general malaise, the law firm said.
"Add that to the fact that almost three quarters of the country's skivers prefer to fake illness on a Monday and that makes today the worst day of the year for employers," said ELAS spokesman Peter Mooney.
But union leaders said workers were particularly vulnerable to illnesses at this time of year and were better off at home if they were sick.
"Most employers are clearly going to prefer that their sick staff stay at home until they are better, rather than become 'mucus troopers' and struggle in, spreading their germs around the office," said TUC general secretary Brendan Barber.