Police are investigating historic sex allegations made against Labour peer Viscount Tonypandy involving a nine-year-old boy.
South Wales Police confirmed it was looking into the claims, which date back to the 1960s and 1970s.
The alleged victim, now reported to be aged 55 and living in Australia, initially contacted police in April 2013.
However, officers had incorrect contact information for him and their investigation was delayed.
The Independent Police Complaints Commission (IPCC) is now assessing the matter, following a referral from South Wales Police.
A spokesman for South Wales Police confirmed: "We are investigating allegations of a sexual assault involving the late George Thomas which is reported to have occurred in the late 1960s and early 1970s.
"It is important that victims have the confidence to come forward and speak to us. We take any allegation involving sexual assault or rape extremely seriously."
The peer, still known as George Thomas at the time of the alleged incident, was a long-standing Speaker of the House of Commons.
Viscount Tonypandy, who worked as a teacher before becoming an MP, died in 1997.
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."
The alleged victim told the Mirror newspaper he was abused by the late MP, who was a friend of his foster parents, at his home and another address in Cardiff.
"I was raped by George Thomas in Cardiff. I was about nine," he told the Mirror. "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."
A spokesman for the IPCC confirmed it was "assessing" the referral by South Wales Police about the delay.
"We have received a referral but we have not made a decision on it, it is still being assessed ," the IPCC spokesman said.
He added that the referral was made on July 16.