An investigation has begun after McDonald's asked its staff to speak English in its restaurants.
The outlet under investigation employs 70 staff
The Commission for Racial Equality (CRE) says it is investigating a notice displayed at one of the fast food outlets in Manchester.
It ordered employees to use English at all times in the store - including the staff room.
McDonald's say the sign was written by a junior member of staff and has since been taken down.
Experts believe the firm could have infringed workers' human rights and European employment law.
The McDonald's staff sign said: "Attention all staff. Due to the common language within the store, all staff members must use English at all times.
"This is in accordance to HQ."
It added: "Warnings can be issued to anyone who doesn't follow this notice. Thankyou."
Complaints by staff were made to Qassim Afzal, a former Manchester city councillor and member of the federal executive of the Liberal Democrat Party, who speaks on equality issues.
He said: "They came to see me. They were really upset.
"They feel they were being deprived of a natural way of expressing themselves. They are British-born people.
"I think it is discriminatory. It is ridiculous."
The investigation centres on the European Directive and the 2000 Race Relations Act and the Equal Human Rights Commission for workers being prevented from speaking in their heritage or native language.
The shop in the Orient food hall of the Trafford shopping centre, employs 70 staff, a third of which are from ethnic minorities.
McDonald's has two outlets in the Trafford Centre
Mr Afzal said McDonald's, as a multi-national company, should be more "sensitive", adding that the use of other languages will help people "respect each other's
languages and differences".
Employment law specialists said the "English language only" rule could be discriminatory - because only someone of English origin could fully comply.
Shop managers at the Manchester outlet referred callers to their London headquarters.
In a statement, McDonald's said: "Within McDonald's we specifically encourage teamwork and inclusivity, and this encompasses the language spoken.
"We have over 70,000 UK staff who speak many languages, representing a diverse, multi-ethnic workforce.
'Need for consistency'
"We recognise however, to ensure consistency that there is a need for a common language and in the UK this is English.
"There are many sensible benefits to having a common language, including consistency in customer service, food quality and safety.
"As a result, staff are encouraged to speak English when working and when liaising with customers.
"Outside of these times we, of course, respect their right to converse in whichever language they choose."
Last week McDonald's launched an internal investigation after poppy-sellers were told to move from one of its restaurants in Ashton-under-Lyne, Greater Manchester, because they were "taking trade".