Schools that allow pupils and staff to wear face veils which hinder learning could be ruled "inadequate", the head of Ofsted has said.
Sir Michael Wilshaw, the chief inspector of schools in England, has given his "full support" to schools and colleges which "decide to take a stand against the inappropriate wearing of the veil".
Education Secretary Nicky Morgan has previously backed schools that want to ban Muslim girls from wearing face veils and suggested teachers should also be barred from wearing the garments.
Sir Michael said Mrs Morgan was right to support schools and other institutions which "insist on removing face coverings when it makes sense to do so".
He said: "I am concerned that some heads and principals who are trying to restrict the wearing of the full veil in certain circumstances are coming under pressure from others to relax their policy.
"I want to assure these leaders that they can rely on my full backing for the stance they are taking.
"I have also made clear to my inspectors that where leaders are condoning the wearing of the face veil by staff members or by pupils when this is clearly hindering communication and effective teaching, they should give consideration to judging the school as inadequate."
Sir Michael's comments were met by dismay by some teaching unions and the Muslim Council of Britain which said Ofsted did not need to resort to "the megaphone of the media to show that it is flexing its muscles".
But the Ofsted chief said he was determined to ensure "discrimination, including on the grounds of gender, has no place in our classrooms".
He added: "We want our schools, whether faith schools or non-faith schools, to prepare their pupils equally for life in 21st century Britain. We need to be confident our children's education and future prospects are not being harmed in any way."
The National Union of Teachers (NUT) said Sir Michael was threatening schools and risked alienating pupils and teaching staff.
Kevin Courtney, the NUT's deputy general secretary, said: "Effective communication between pupils and staff is essential to effective teaching and learning.
"However, Sir Michael Wilshaw once again has chosen to issue punitive diktats to threaten schools through the use of 'inadequate' Ofsted judgments, rather than enabling them to develop their own sensible and appropriate policies on the wearing of religious clothing at school.
"Rather than assisting school leaders, this will have the effect of alienating many staff and pupils."
Last week Sir Michael told BBC2's Newsnight that he would back banning veils and said school inspectors had found the covers were causing communication problems in the classroom on occasions.
Prime Minister David Cameron has said he would back institutions that have "sensible rules" over Muslims wearing full-face veils but ruled out imposing a French-style ban on full-face veils in public.
A spokeswoman for the Muslim Council of Britain: "We are a country that prides itself in accommodation and fair play. It is a shame that the niqab - the full face veil that a minority of Muslim women wear - has become a polarising issue when it need not be.
"Accommodation can also be made around the niqab as well and Ofsted need not have resorted to the megaphone of the media to show that it is flexing its muscles."
NASUWT general secretary Chris Keates said: "Schools will now apparently be judged inadequate on the basis of an inspector taking exception to a pupil or member of staff wearing a veil and deeming it a barrier to communication.
"Ofsted should be driven by evidence. Where is the evidence that demonstrates that wearing the veil is a barrier to teaching and learning?"
A Department for Education spokesman said: "We fully support Sir Michael's statement. We are pleased that Heads and school leaders who choose to implement policies which restrict the wearing of the veil to support effective teaching and learning will receive Ofsted's backing.
"It is also clearly right that if the wearing of the veil is interfering with education in schools that should trigger action from Ofsted."