Police are investigating an "appalling" seven-year fraud worth around £914,000 at a top federation of schools, it has finally been admitted - but parents have been gagged from talking about it.

The chief executive of Haberdashers' Aske's Federation Trust publicly admitted the investigation's existence in a letter to parents last month, but the organisation has maintained silence over many of the details. It is understood the missing sum is £914,000 and goes back seven years to 2005 when the trust was set up.

Along with the flagship Hatcham College in New Cross, Downham's Knight's Academy and Crayford Academy, the federation has also set up the first free school in Lewisham borough - Hatcham Temple Grove.

Parents are shocked by the news - which went largely unnoticed in accounts published last year - but those who were approached by News Shopper refused to comment after a warning letter went out from the federation.

In his letter, Adrian Percival told parents: "In October 2012, we discovered that a serious financial fraud had been committed by an employee of the Haberdashers’ Aske’s Federation, the body that manages each of the Academy schools. Put simply, we believe this employee used their position of trust in the finance team to steal a substantial amount of money from the schools.

"This appalling crime appears to have been committed over a period of seven years, during which time the perpetrator appears to have set up a large number of small fraudulent transactions in order to commit these crimes unnoticed.

"As you would expect, we immediately suspended and subsequently dismissed the employee concerned and launched an independent investigation to ascertain how this could have happened and the amount that had been stolen from us."

Mr Percival said: "The civil courts have already found in favour of the federation, and we are now taking action to reclaim as much of the money stolen from us as we can by seizing the offender’s assets."

And the letter warned parents: "Given the ongoing nature of the criminal investigation it is very important that nobody comments publicly on this situation, because this might jeopardise the success of any criminal trial that may take place.

"If you receive any enquiries from the media, I should be most grateful if you could refer them to me without offering any comment and we will deal with them appropriately."

The Met Police have confirmed an investigation is ongoing and that a 55-year-old man from Lambeth was arrested in October 2012. He remains on police bail and is due to return to a police station later this month.

A spokesman for the federation would only say: "The civil case found in favour of the federation, and we are in the process of trying to recover the money that has been taken from us. We are not able to confirm the amount that has been taken, but we are obviously shocked and saddened that this has happened to the federation."