A supply teacher agency claims that some schools on the outskirts of London seem reluctant to hire ethnic minority temporary staff.
The London Teaching Pool is complaining about inquiries from schools appearing not to want non-white teachers.
Managing director Darryl Mydat says such requests are received by his firm several times each term.
Head teachers' leader Brian Lightman said such discrimination by schools was "inconceivable".
The owner of another supply teachers' agency said schools might have legitimate concerns about English language skills - and that this might be misinterpreted as racism.
But Mr Mydat says his recruitment firm has seen a disturbing number of cases where schools signal that they would prefer not to have ethnic minority supply teachers.
He says the request might use a phrase such as "We want a 'John Smith', if you know what I mean."
But he claims there is a clear impression that what they want is a "white, Anglo-Saxon teacher".
Mr Mydat says this seems to be a particular issue in schools on the fringes of London.
He says it might suggest schools are concerned about how classes in schools with few ethnic minority pupils would react to a black or Asian supply teacher.
Mr Mydat says his business might lose some work as a result of his claims - but says there is an important principle at stake.
"I'm appalled by it. There shouldn't be any place for it."
He says that when such requests are received, his firm has stopped supplying teachers to the school - but he does not want to identify these particular schools.
Mr Lightman, general secretary of the Association of School and College Leaders, said he had never heard of such a problem and doubted whether schools would behave in this way.
"I find it inconceivable to think a school would discriminate on grounds of race."
But the managing director of another teacher recruitment agency operating in several different parts of England, who did not want to be identified, said that the issues of race and ethnicity could be confused with English language skills.
"Language is the issue, not race," he said.
He said schools could be "quite blunt" about what they wanted from supply agencies - and that could include not wanting someone who did not speak good English.
"The schools will say: 'I need someone the kids are going to be able to understand. If they can't understand him, the kids are going to play up and we're going to have hassle.' They're quite frank about telling you."
And such schools might check with an agency if a teacher's name suggested they might not speak English as a first language, he said.
"If it's a foreign-sounding name they might ring back and ask what's his English like?" says the owner of a recruitment firm, who was once a teacher himself.
He also said that some schools wanted to see photographs of temporary teachers before they hired them.
And he recognised that teachers could sometimes face a negative reaction. A white female teacher working for his agency had hostility from parents in a school serving Muslim pupils, he said.
But he did not believe that schools were unfairly discriminating against teachers.
"Schools are pre-empting it and saying if their English isn't good enough, we don't want them in. But I've never experienced them saying I don't want you because you're black or white or yellow or whatever," said the recruitment firm owner, who has more than a decade's experience in providing supply teachers.
"Language is an issue... kids are going to look for an opportunity to play up. That's what drives it. When you have a supply teacher, you don't want to hold their hand. You want them to get on with it. You don't want them sending kids to you saying they've made fun of your accent."
The head of another supply teacher agency suggested that such problems might have existed in the past, but would not be tolerated now.
There were other recruitment agencies who declined to comment.
Mr Mydat said his claims about some schools not wanting ethnic minority supply teachers are against a background in which most schools have become much more professional about hiring supply teachers.
He says that schools, with more independence and control over their own finance, are much more focused on getting the right supply teachers.
The Nasuwt union last year published a survey of teachers' experiences of supply work. It found that ethnic minority teachers had more difficulty in finding work than the average for all teachers.
They were also less likely to feel welcome in the staff room. There were 14% of ethnic minority supply staff who said they were "rarely made to feel welcome" in schools, compared with 8% for all staff.
They were also disproportionately likely to be underpaid, in relation to their previous experience.