Education Secretary Nicky Morgan labelled a teacher "sexist" after he questioned whether she was really in charge of her department.
Morgan responded sharply to a question from Simon Kidwell, the headteacher of Hartford Manor Primary School in Cheshire, after she gave a speech at the National Association of Head Teachers (NAHT) in Birmingham.
He asked: "Are you in charge of the department, or is Nick Gibb?"
The question was met with a round of applause and cheers from the headteachers, but Morgan was not impressed.
When the clapping eased, she replied: "I am not going to dignify that sexist remark with a comment."
Russell Hobby, general secretary of the NAHT, said he believed the question stemmed from Gibb's involvement in decision making in his capacity as Schools Minister but said he did not think Gibb was running the department.
He said he had "no idea" why Morgan thought the question was sexist.
Christine Blower, general secretary of the National Union of Teachers, said she could not see how the question could be classed as sexist.
Speaking after Morgan's speech, she said: "There clearly is a lot of sexism in the world but I wouldn't have thought that anyone challenging her when Nick Gibb has been very high profile on the issues to do with assessment could possibly count as a piece of sexism.
Blower said she thought it was a "completely valid" question as she thought Gibb had apologised for the "chaos" schools were in, rather than Morgan.
"It seems to me that if something goes spectacularly wrong, like the putting of that test on the website then it should be the person, hierarchical though it is, at the top who apologises and not her deputy, so I don't think this is about sexism - this is about the buck stopping where the buck should stop.
"It would be a very good thing if she acknowledged that frankly at the moment there's a significant amount of chaos in the curriculum and in assessment."
Blower added that there was a general sense of no engagement from the education secretary.
She said that several head teachers shouted during the speech that Morgan was not listening.