Sixty prisoners are "actively involved" in the disturbance at HMP Ranby after twice as many inmates refused to return to their cells, the Prison Officers' Association (POA) has said.
At around 12.30pm 120 prisoners refused to return to their cells. They have taken control of a unit.
We have got national resources at the prison trying to establish a surrender plan. We know 60 prisoners have been actively involved.
Mr Travis said today's disturbance was "no surprise" to the POA.
It's all down to chronic staff shortfalls and a management that are hell-bent on delivering things they can't deliver safely.
Prisoners are saying 'We're not dealing with this'. They will react.
More top news
The major party leaders' campaigns for the General Election will remain suspended on Wednesday following the Manchester terror attack.
Thousands of people have donated to help the families of the victims - as well as a homeless man who came to their aid.
Some children, particularly those who have been more directly affected, may show symptoms of stress and trauma.