Fourteen crimes involving adults working in positions of trust having sex with 16 or 17-year-olds in their care have been recorded in Northamptonshire since 2012.
It is illegal for teachers, care workers and youth justice staff to have sexual contact with 16 and 17-year-old children under their supervision.
Since 2012 there has been a 51 per cent increase in abuse of position of trust offences, and the NSPCC believes a loophole in the law exists which means children are not properly protected by the likes of sports coaches, religious leaders and youth workers.
The children's charity's chief executive has criticised the Government for backtracking after sports minister Tracey Crouch announced in November 2017 her department had agreed to extend the law to sports coaches.
The Ministry of Justice wrote to the NSPCC making clear that the Government believes laws on the age of consent and on non-consensual sexual activity provide adequate protection for 16 and 17-year-olds who are preyed upon by adults who supervise them.
"This change in direction is as disappointing as it is dangerous," said NSPCC chief executive Peter Wanless.
"It shows a lack of understanding of the nature of grooming whereby young people might feel that they are in a loving relationship, when in fact an adult with considerable power and influence over them is abusing their position of trust for sexual gratification.
"That position of power is not diminished if it exists on the sports pitch, in the minibus, or in the changing rooms, as opposed to in the classroom. Yet bizarrely the law draws such a distinction."
Mr Wanless added: "More than 1,400 of these crimes have been recorded in recent years, but this could be just the tip of the iceberg given the plethora of roles not covered.
"How many more hundreds of children will be abused before the Government delivers on its promise made in the House of Commons?"
Television presenter and campaigner Charlie Webster, who was abused by her running coach when she was 15, echoed Mr Wanless and blasted the Government for this change in direction.
Ms Webster said: "I, along with the NSPCC, have repeatedly highlighted and investigated this loophole for several years now.
"I was told after numerous meetings where I brought its danger to light that we had been successful in amending the law.
"It is not only a disappointment that this has happened but it scares me that we are still letting our most vulnerable down as they slip through the net.
"Despite the fact that sexual abuse and the grooming process has become a narrative more out in the open the law and its implementation is still regressive.
"I can’t emphasis enough how dangerous this is."
The NSPCC is calling for the Government to deliver on its promise to extend position of trust laws to sports coaches.
The charity will continue to campaign for laws on positions of trust to be extended to cover all adults defined as working in what is known as 'regulated activity'.
This applies to adults who work regularly with children, and who teach, train, instruct, care for or supervise children, or provide guidance on well-being, or drive a vehicle only for children.
By using this definition, the law will not be limited to an arbitrary list of settings that inevitably leaves some children unprotected.