The chief executive of a Norfolk academy chain at the centre of allegations about school inspection tip-offs has broken her silence in a letter to staff that strongly denies the claims.

Dame Rachel de Souza, chief executive of the Inspiration Trust, and its chairman, Theodore Agnew, responded to renewed claims in a Sunday newspaper that three schools connected to Dame Rachel received advance notice of the dates that Ofsted inspectors would visit.

The schools were Ormiston Victory Academy, which Dame Rachel led before taking up her current role, and Great Yarmouth Primary Academy and Thetford Academy, which are members of the Inspiration Trust.

In their most categorical denial of the allegations to date, the letter, sent today, said: 'We want to reassure you that these serious allegations are absolutely false.'

The letter continued: 'To be very clear, neither Rachel nor anybody at the schools concerned, nor anybody at The Inspiration Trust, has ever been tipped off about inspection dates. It would be disgraceful if that were true. It is not true.

'In two of the cases the Observer has mentioned – at Ormiston Victory Academy and at Great Yarmouth Primary Academy – we simply did what schools do up and down the land. We guessed that Ofsted were due to inspect because there is a narrow window of time in which this must happen.

'The nearer we got to this time slot expiring the more obvious it became that an inspection was imminent. The emails the Observer has somehow obtained and published merely reflect that and are not based on inside information of any kind.'

Last month, a report by Sir Robin Bosher, Ofsted's director of quality and training, found 'no evidence to substantiate the allegations that the three schools in question had improperly received prior notification of the dates of their Ofsted inspections in order to put them at an unfair advantage'.

On Monday, Ofsted said it stood by Sir Robin's findings.

Today's letter to Inspiration staff said that, as Sir Robin found, Dame Rachel was erroneously sent the date of a scheduled inspection of Great Yarmouth Primary Academy during her training to be a part-time Ofsted inspector, but the date was 'immediately and confidentially changed by Ofsted'.

It added that Thetford Academy staff used 'reasonable guesswork' about an imminent inspection.