Long-awaited new powers to tackle paedophiles who target children through social media or mobile phones will take effect on Monday.
Groomers convicted of sexual communication with a child will face up to two years in prison and automatic inclusion on the sex offenders' register.
The offence aims to cover a gap in existing legislation and allow police and prosecutors to intervene earlier and stamp out grooming before sexual activity can take place.
Ministers have faced criticism over a delay in implementing the measure, Section 67 of the Serious Crime Act 2015, which has been on the statute book for more than two years.
The legislation was given royal assent in March 2015 but it was not until last month that the Government specified a date for the anti-grooming clause to come into force.
Justice Secretary Elizabeth Truss said: "In a world of mobile phones and social media, our children are ever more vulnerable to those who prey on their innocence and exploit their trust.
"The best way of protecting our young people from the evils of child abuse is to stop it happening in the first place.
"This new offence will give courts the powers to jail anyone who sends a sexual communication to a child and stop the process of grooming before it starts."
The MoJ said Ms Truss made enacting the clause a priority, and has "acted decisively to ensure children are protected both online and offline".
The NSPCC, which had previously accused the Government of "dragging its feet" over the law, welcomed the development.
Peter Wanless, the charity's chief executive, said: "The Justice Secretary has done the right thing. It is a victory for common sense. Children should be as safe online as they are offline, wherever they are in the UK.
"This law will give police in England and Wales the powers they need to protect children from online grooming, and to intervene sooner to stop abuse before it starts."
It is a criminal offence under the new law for someone aged 18 or over to intentionally communicate with a child under 16, where the perpetrator acts for a sexual purpose and the communication is sexual or intended to elicit a sexual response.
The offence applies to both online and offline contact including social media, texts and letters.
Officials said, while there are a range of offences already available to deal with sexual communication, many do not automatically attract sex offender registration.
Victims' Commissioner Baroness Newlove said: "We need to make sure we are doing everything possible to protect children from online grooming and predators.
"The new law to prevent sexual communication with a child will help keep children safe in a digital world and prevent future victims."