He was jailed in 1999 for 14 months after admitting unlawful sex with a 13 year-old girl who said he had spiked her drink and threatened to kill her if she told anyone about the incident, a police spokeswoman said. She added that his catalogue of offending goes back to 1980.
Detective Chief Inspector Ian Dawson, of Lancashire police, said: "Leach is an extremely dangerous and predatory individual who poses a serious threat to members of the public.
"He has previous convictions for burglary, sex offences against a young girl, assault and robberies in which he has targeted elderly people in their own homes.
"We believe he could be sleeping rough and travelling extensively throughout the country. He has a number of links to Preston and prior to his sentence lived and offended in the Preston area.
"We are liaising with a number of other forces in a bid to track him down and apprehend him but we need the public's help."
He added: "I would remind people that he is violent and should not be approached.
"The best possible outcome for himself, his family and all concerned is that he gives himself up."
Leach was jailed in 2005 for robbing a 90 year-old woman. He was given a life sentence with a minimum tariff of five years. He also has convictions for burglary and robbery and assault.
Previously, in 1984 he was jailed for seven years for robbing and burgling a 79 year-old woman in her sheltered accommodation flat, and striking her in the face and body with his fists.
He has several other convictions for targeting elderly victims.
He also wounded a man and woman with a knife after an argument in 2001. In 1995 he was fined £150 for flashing a woman jogger.
The offender is described as white, around five feet 11 inches tall, of stocky build with cropped hair and hazel eyes.
Detectives believe Leach may now have stubble or have grown a beard. He could also be carrying a rucksack.
Leach has links to Preston but police believe he could be anywhere in the country. Police are also pursuing lines of inquiry in Nottinghamshire.