Malcolm Millman, 58, has failed to return to a medium-security mental heath facility in Milton Keynes after a day visit to a monastery near Hemel Hampstead.
He is thought to have taken two trains; one into London from Hemel Hampstead, and then a second train into Leeds.
Millman was last seen at Manchester Airport, Thames Valley Police said, although they do not believe he has access to a passport.
Due to his previous convictions, including rape, police believe he is a potential risk to the public.
Super Intendant Olly Wright of Thames Valley Police said: "We are continuing our appeal for information to trace absconder Malcolm Millman who failed to return to a medium security facility in Milton Keynes.
“Millman was convicted of rape in 1996 and was due to serve twelve years, however he was working towards his full release while under the care at Chadwick Lodge.
“Based on Millman's previous convictions, including rape, we believe he is a potential risk to the public.”
He added: “If any members of the public see Millman do not approach him, instead call 999."
Police have released CCTV images of Millman, in a bid to help the public to identify him.
The 58-year-old, who uses a walking stick, is described as white, of large build, bald with thinning brown hair on the sides and brown eyes.
He was last seen wearing a brown jacket, purple jumper, grey tracksuit bottoms, blue and white trainers and carrying a walking stick and is also known by the names Richard Simon Barber, Michael Geoffrey Newman, and Raymond Pilkington.
Police have also said he has links to Kent, and Canterbury specifically.
If you are reading this and have any information whatsoever about Millman’s whereabouts then please contact police immediately.
If you don’t feel comfortable speaking directly to the police then you can contact the independent charity Crimestoppers anonymously on 0800 555 111 or online at http://www.crimestoppers-uk.org.
No personal details are taken, information is not traced or recorded and you will not go to court.