Carvel Bennett admitted having sexual intercourse with the young teenager in the 1970s and accepted he is the biological father of her daughter, Birmingham Crown Court heard.
But the 74-year-old, from Hansons Bridge Road, Erdington, denied a charge of rape claiming the girl told him she was 16 and consented to sex.
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After a trial, a jury took just one hour and 48 minutes to reject his account and find him guilty.
The full circumstances of how Bennett came into contact with the girl cannot be reported in order to protect the victim’s legal right to anonymity.
Opening the case on Thursday, July 29, Prosecutor Peter Glenser QC stated the defendant told her to take her clothes off when they were alone together.
“She does recall saying ‘I don’t want to do this’ and him saying ‘it’s going to be alright’,” said Mr Glenser who added: “She remembers him saying he wasn’t going to hurt her and she should say nothing.”
The prosecutor revealed that a few weeks later it ‘became obvious she was pregnant’ and confirmed the victim ultimately gave birth to a baby girl who was put up for adoption.
The jury heard when the child reached the age of 18 she obtained some social security records and tracked down her mother who revealed how she was conceived.
When she acquired her full records she saw that Bennett was named as her father, said Mr Glenser, who went on to say that DNA results suggested it was 22 million times more likely the defendant was her dad than any other unknown Afro-Carribean man unrelated to him.
The prosecutor added: “In reality, there is little in dispute in this case save for the central and important issue of consent.”
In an interview played to the court the victim stated Bennett did not use any violence, but that she ‘scared’ to resist his attack.
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Tom Walkling, defending Bennett, requested he be given the chance to put his affairs in order arguing he will likely spend the rest of his life in prison, given his age.
But Judge Martin Hurst stated ‘the risk he might do something foolish is too great’ and remanded him into custody.
Sentencing will take place on August 3.
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