The Health Protection Agency East Midlands launched an investigation in September after three people at The Kingswood School, Corby, contracted tuberculosis.

More than 200 staff and pupils were screened because of the outbreak.

But in documents Dr Stephen Horsley, director of public health at NHS Northamptonshire, says a total of seven cases have been identified as positive to date.

Pupils in Year 12 at the school will now be screened – including those who tested negative when initial testing was carried out.

In a report due to be presented to NHS Northamptonshire's public board meeting tomorrow, Dr Horsley said: "Three students in Year 12 were diagnosed with pulmonary TB at The Kingswood School in Corby.

"As a result of these cases and following a risk assessment made around each case and the school environment, a decision was made to screen the whole of this year group, plus Year 13 of 2008-09 (who have now left school) and all teachers with links to the school for Years 12 and 13.

"To date, seven cases have been identified as positive and as a result students in Year 12 will now be screened and those who tested negative previously will have to be rescreened.

"The Health Protection Agency is meeting regularly with key people to monitor the outbreak."

It was revealed in October that blood tests showed a total of 57 people had been exposed to tuberculosis after three full-blown cases at school.

The 57 positive blood tests meant those concerned had been exposed to TB but did not have the disease.