Prison officers have foiled a possible escape plot at one of the UK's most secure jails, the Ministry of Justice has confirmed.
An investigation is under way into the incident at Full Sutton prison, near York, which was discovered on Friday.
But officials could not confirm reports the plot involved convicted terrorists getting out by using a helicopter.
The report said jail staff foiled the plot just hours before it was due to take place.
A Ministry of Justice spokeswoman said: "Vigilant staff at HMP Full Sutton uncovered details of a possible escape plot on Friday.
"The prisoners involved were immediately separated and the Prison Service is investigating the matter."
Full Sutton is a maximum security prison which houses more than 600 men, including many lifers.
The only successful prison break using a helicopter in the UK was more than 20 years ago when two inmates escaped from Gartree Prison, in Leicestershire.