Leigh Arendse, 25, of Brunswick Park, London, groomed the innocent youngster on an internet chat room before going from London to her home to meet her.
He then spent 48 hours in her bedroom watching DVDs and having séx with her - and hid behind the door when her mum went to check on her.
Sick Arendse claimed he thought the girl was actually 14 and only realised her true age when her mum told her off for smoking in her room.
He then left the house and went to London with the girl whose parents raised the alarm when she went missing last August.
Arendse has now been jailed for four years after admitting two counts of raping the girl.
Judge Jonathan Haworth told him: 'This is not a case of an 18 or 19-year-old - you are a mature adult.
'You had been in her bedroom when you knew that members of her family were also in the house.
'There was a sheer irresponsibility on your part when you failed to find out her real age.'
Sara Walker, prosecuting, told Cambridge Crown Court: 'The defendant stayed in the victim's room where the two watched DVDs.
'The next evening, her mum came up and he hid behind the door.
'Her mum was appalled at the mess and said that her daughter shouldn't be smoking at the age of 12. The defendant then found out that she was indeed 12.'
She added: 'The victim said she started to kiss him and that he initially pushed her off.
'She then tried to kiss him again and he kissed her back before they had séx.'
The court heard that the youngster told her family she had unprotected séx with Arendse.
Defense claimed that Arendse's response to finding out the girl's true age was one of 'shock' and he decided to return to London.
He said: 'She was the one who insisted on following him to London and in his defence he didn't want her to come with him.'
Arendse was also handed a 10-year Séxual Offences Prevention Order which bans him from having unsupervised contact with children under 16, possessing any device capable of accessing the internet unless it retains and displays history and deleting such history.
He was also placed on the séx offenders' register .
Police said: 'Arendse was clearly aware that the girl in this case was underage but continued his contact with her and arranged to meet up.
'He had plenty of opportunities to stop the relationship but chose to travel to meet her to have séx.
'This case shows the dangers of using online chat rooms and how easily perpetrators can gain the confidence of young people in order to exploit them.'
Source: Dailymail
