A landmark case which could give new rights to millions of carers may affect workplace relations, according to the Federation of Small Businesses (FSB).
Sharon Coleman's case will now go to a panel of European judges
Sharon Coleman, who has a disabled son, has won the initial stages of a case at the European Court of Justice claiming "discrimination by association".
It has led to calls for employers to look at their flexible working arrangements for carers.
But the FSB warns it could cause "resentment" between workers.
"This may sour industrial relations in the workplace," said Stephen Alambritis, chief spokesman for the FSB.
Legal secretary Ms Coleman claimed that she was described as "lazy" for wanting time off to care for her disabled son, and was not allowed as much flexibility as other parents by her former employers Attridge Law in London.
As primary carer, Ms Coleman wanted flexible working arrangements, but accepted voluntary redundancy and began a claim for constructive dismissal five months later.
An employment tribunal hearing the case referred it to the European Court for a ruling on whether EU discrimination laws also applied to people closely associated with a disabled person.
Advocate-General Poiares Maduro said that a seven-year-old EU law protected people who, although not disabled themselves, suffered discrimination as a result of their close association with a disabled person.
"One way of undermining the dignity and autonomy of people... is to target not them, but third persons who are closely associated with them," he said.
A panel of European judges will make a final ruling later this year, but employers are already being encouraged to be aware of the potential knock-on effect.
The FSB said 90% of requests for flexible working were already agreed to by employers, but they would now need more help and guidance from the Government and the Equality and Human Rights Commission.
Mr Alambritis said that bosses would need help to explain legislation to staff instead of being accused of favouritism when carers took time off.
"Sometimes resentment from other staff has to be dealt with," he said.
The issues were particularly acute for employers in small businesses, who were usually as flexible as they could be. Other staff often had to pick up colleagues' work, he said.
"The opinion in this case has been eagerly anticipated, as the knock-on effect could be quite significant," said Rachel Dineley, employment partner and head of the national discrimination unit at commercial law firm Beachcroft LLP.
"Until now, it has not been clear whether you can claim direct discrimination by association in relation to disability - this had only been established when it came to race discrimination.
"The Advocate-General makes clear that in his view, these same principles would equally apply to direct discrimination or harassment, by association, on grounds of religion or belief, age or sexual orientation."
She added that, if confirmed by the final ruling, the case could lead to a significant rise in claims of this kind.