A judge has warned parents of the dangers of on-line sexual predators after jailing a 26-year-old Essex man for more than seven years after he posed as a teenager on Facebook and persuaded four girls to send him naked pictures of themselves.

Sentencing Tom Willsher, Judge Martyn Levett branded him a "danger to young women" and said the case highlighted the need for vigilance by parents at a time when so many children had access to smart phones, computers and tablets.

"This case demonstrates a continuing problem and demonstrates to the public the great care parents need to take with their children," said the judge.

Ipswich Crown Court heard that Willsher threatened to post the pictures of one of the girls on social media and threatened to display naked pictures of another girl at her school when they tried to stop contact with him.

Simon Gladwell, prosecuting, said one of the girls, who was 13, had been so "petrified" that Willsher was going to carry out his threat she had been prescribed medication to deal with issues resulting from what happened.

The court heard that in addition to pressuring the girls to send him naked pictures of themselves he had also sent them intimate pictures of himself.

In a statement read to the court one of the victims said she had suffered nightmares as a result of the offences and felt ashamed.

Another victim said she had been left feeling "ugly and dirty" as a result of the offences and had self-harmed and tried to take her own life.

Willsher, 26, of Chapel Road, Brightlingsea, admitted attempting to meet a girl with intent to have sex with her and three offences of breaching a sexual harm prevention order.

He also admitted two offences of causing or inciting a child to engage in sexual activity, causing a female over 16 to engage in sexual activity and possessing indecent images of a child.

Willsher was jailed for seven years and eight months with an extended licence period of two years and four months.

He was also ordered to sign the sex offenders' register for life and was made the subject of an indefinite sexual harm prevention order.

Edward Duncan-Smith, for Willsher, said his client had been diagnosed with low spectrum autism, ADHD and learning difficulties.