Ofsted has had to apologise for an “unacceptable” breach of confidentiality after it wrote to the wrong school about allegations of cheating in Sats.

Richard Greenfield, head of St Edmund’s Catholic Primary in Edmonton, north London, was shocked to receive a letter from the watchdog about a claim he had helped pupils in a maths test by repeating questions several times.

The letter was copied to both his chair of governors and former chair of governors but, it turned out, did not relate to his school at all.

“It is such a basic mistake considering they had taken the trouble to find out our names,” said Dr Greenfield. “Apart from the inefficiency of it, my concern is that these things can get into the public domain and cause a lot of trouble and harm. Mud sticks.”

He said the letters were not marked confidential and came into the school office where they were opened by a member of support staff. They included full details of the anonymous allegation and explained it would be passed on to the local authority.

“It is the gravity of the mistake that has been made by an official body that judges and condemns other people that annoys me,” the head said. “I wonder how forgiving they would be of a school if the positions were reversed?”

Christine Gilbert, chief inspector, has written to Dr Greenfield to “apologise unreservedly” for the “unacceptable” breach of confidentiality.

“You can be assured that appropriate action through our disciplinary procedures has been taken in respect of the member of staff responsible for this administrative error,” her letter says.

An Ofsted spokesman said: “We have apologised and assured the school that this will not impact on them in any manner. We take this matter seriously and are reviewing our... processes to ensure such a situation does not happen again.’