An armed robber and another inmate are on the run from Leyhill open prison in South Gloucestershire, police said.
Stephen Gurney, 43, from Chelsea, was serving an indeterminate sentence for armed robbery and Marcus McAlpine, 43, from Lewisham, jailed for assault.
The men failed to appear for roll call on Sunday evening and police said they were spotted walking through North Nibley heading towards Cam earlier.
Police said neither man should be approached by the public.
An Avon and Somerset Police spokesperson said it is thought they absconded together but it is possible they may have now split up.
Stephen Gurney is 5ft 8ins tall, of average build with 21 tattoos over his body and legs.
Marcus McAlpine is 6ft 1ins tall, with a tribal tattoo left arm.
Anyone spotting the men is asked to call the police immediately.