A new legal ruling on sick leave and holidays could allow unscrupulous workers to cheat their bosses, says the CBI employers' group.
The European Court of Justice decision means workers taken ill on holiday could claim time back as sick leave.
But the CBI said the judgement was "open to abuse" by employees who might see it as an easy way of increasing their holiday entitlement.
The ruling stems from a case brought by a Spaniard working for Madrid council.
The employee, Francisco Pereda, began his legal battle two years ago against Madrid Movilidad SA, after being refused the right to reschedule his summer holiday because of an injury he had recently suffered.
The court in Luxembourg decided that under the EU's Working Time Directive, he should have been allowed to change his holiday dates and could not be forced "to take annual leave during a period of sick leave".
Lawyers examining the ruling said it could make it possible for workers already on holiday to phone in as unwell - in effect, giving them extra time off. The ruling is binding on the 27 EU member states.
Katja Hall of the Confederation of British Industry (CBI) said: "Many firms already take a common-sense and sympathetic approach. But allowing employees to reclassify their holiday as sick leave opens the door to abuse.
"If this ruling is to become law, employers should be able to ask for a medical certificate."
Article Seven of the EU's Working Time Directive guarantees every worker paid annual leave of at least four weeks.
Interpreting the directive, the court said that, in cases where annual leave coincides with sick leave, Article Seven overrides any national or collective agreement that prevents a worker rescheduling his annual leave.
The court ruling did not specify when the worker would have to report his or her sickness to the employer. Nor did it say at what time an illness could trigger the right to alter holidays.
'Not from real world'
A UK human resources institute, the Chartered Institute of Personnel and Development (CIPD), said the ruling "is not from the real world, and risks forcing good employers to ditch relatively generous occupational sick pay schemes, and opt for less generous Statutory Sick Pay schemes instead".
In light of the ruling, the CIPD said, employers should ensure that employees "phone their line manager daily if they are sick while on holiday, to confirm that they are still sick and not fit for work".
The EU judgement comes as lawyers are still analysing the implications of another decision on a related issue made by the House of Lords earlier this year.
Three months ago, the Law Lords found in favour of a group of former UK Customs staff, ruling that workers who were on long-term sick leave could, in certain circumstances, accumulate holiday time and even carry it forward, rather than losing it at the end of the year.