Police say they are investigating allegations the former House of Commons speaker George Thomas sexually abused a nine-year-old boy.
The alleged victim, now aged 55, told the Mirror newspaper he was abused in the 1960s and 1970s by the late MP who was a friend of his foster parents.
South Wales Police admitted the claims were first made in 2013 and they failed at first to contact the complainant.
The force has referred the matter to the police watchdog, the IPCC.
The alleged victim, who now lives in Australia, told the newspaper he was raped by Thomas, who later became Viscount Tonypandy, at his home and another address in Cardiff.
"He spent a lot of time at my house as my parents were good friends with him. Things started small but then got a lot worse. It has been with me all my life," he said.
He claims he reported the abuse to South Wales Police on two occasions but was "disappointed" with the response from officers.
South Wales Police Assistant Chief Constable Nikki Holland said: "We were made aware of these allegations in April 2013 and have attempted to get in touch with the victim.
"Unfortunately, incorrect contact information was used and as a result we failed to make contact with him. We have since spoken to the victim, apologised for the delay and are investigating his claims.
"This delay was clearly unacceptable and we have referred the matter to the IPCC.
"It is important that victims have the confidence to come forward and speak to us. We take any allegations regarding sexual assault or rape extremely seriously and urge anyone who has been a victim of this type of crime to make contact with us."
A spokesman for the Independent Police Complaints Commission (IPCC) confirmed it had received a referral from the force and was assessing it.
Viscount Tonypandy, who was a Labour MP in Cardiff from 1945 to 1983 died of cancer in 1997, aged 88.
The Methodist preacher held the role of Secretary of State for Wales from 1968 to 1970 and was Commons Speaker between 1976 and 1983.