One of Oxford's most dangerous sex offenders has been banned from any contact with children forever.

David Cullen - who was put behind bars 11 years ago for sexually abusing a child - has been jailed again after befriending a woman and then grooming and molesting her young son.

Judge Julian Hall gave the 47-year-old an indeterminent sentence and imposed a Sexual Offences Prevention Order (SOPO) banning him from contact with children, even if their parents are present.

Cullen, formerly of Blackbird Leys in Oxford, admitted four counts of sexual assault against a person under the age of 13 and twice breaching an interim SOPO.

Sarah Gaunt, prosecuting, told Judge Hall at Oxford Crown Court that Cullen had befriended his victim's mother before abusing him between January 30 and June 30 last year.

Miss Gaunt said: "Mr Cullen told the boy not to tell his mother or she would tell him off."

On June 29, 2006, the boy broke down in tears and informed his mother of the sexual abuse, leading to Cullen's arrest the next day.

Cullen was given an interim order in July last year, which prevented him from coming into contact with children under 16.

He breached the conditions of this on two separate occasions.

The first instance was on August 17 when he was seen with two boys in the Sports World shop at the Oxford Retail Park, Cowley.

Witnesses saw Cullen talking to the boys and driving off with them.

The defence claimed Cullen had bumped into them while shopping, but when arrested later it was found he had spent £174.30 on the two boys, something Miss Gaunt said was a clear sign of grooming.

He was given a second interim SOPO. But between January 31 and February 7 this year, Cullen was seen in the Harbour Light Amusements, in Fleetwood, Lancashire, talking to children.

Miss Gaunt said: "Mr Cullen was seen in a car with a group of children waving wads of cash around in front of them."

According to the prosecution, the manager of the arcade said this was "just not right" and instructed staff to keep an eye on Cullen.

Judge Hall said: "You've done this before, that is one of the evils in your case. What's more you abused the trust of the mother you befriended."

He continued: "Part of it is down to a cycle of abuse you were subjected to as a young man, but I have got to stop you doing it again."

Cullen was sentenced to a minimum of two years and three months in prison, placed on the sex offenders register and given a full SOPO. The parole board must be satisfied he is no longer a serious risk to children before he is released.

There are 19 people under SOPOs and 936 people on the sex offenders' register in Oxfordshire.

Police risk assessment co-ordinator for Oxford, Keith Ringsell, said: "David Cullen had a lot of help in his life but he chose to reoffend, which is very disappointing."