The health, safety and futures of the country’s most vulnerable children are being damaged by the gap between child and adult services, the UK’s main children’s charities have said.

Unlike children in care, support for children in need stops at 18. But as few as 3% of vulnerable teenagers are referred to adult services. There is no requirement for local authorities to put in place a plan to help children transition to adulthood.

The Children’s Society is campaigning with 12 other children’s charities for a cross-governmental body to develop a coordinated response for vulnerable 16- and 17-year-olds who are in need – but not taken into care – because of domestic violence, mental health, substance misuse, going missing from home, sexual and other forms of exploitation.

In a letter to the Guardian, the charity announces its new campaign, Transitions to Adulthood, being launched on Tuesday at an event at the House of Commons, supported by about 35 MPs and Lords.

Vicky Ford, the Conservative MP for Chelmsford, said: “Vulnerable young people, especially 16- and 17-year-olds is at the heart of some of the biggest social policy issues we face today such as child sexual exploitation or being drawn into youth violence or homelessness,

“Evidence shows us that where local authorities and other agencies work together to identify vulnerable children and intervene early, this can have a dramatic and positive improvement on the lives of young people,” she added. “It also reduces long-term costs to the government.”

Hannah, now 23, was turned away by both children’s and adult services when she was 17. “My GP was repeatedly told that I was too old for children services but too young for adult services,” she said. “I was desperate – self harming and attempting to take my own life – but there was nothing for me: children’s services said that by the time they had assessed me and decided on a treatment plan, I would have been too old for their help so they refused to touch me.”

Nick Roseveare, the chief executive of the Children’s Society, which organised the event and has been campaigning to improve support for vulnerable 16- and 17-year-olds through its Seriously Awkward campaign, said: “Too often, support for young people living complex lives falls short and then all of a sudden disappears on the day they turn 18, while they continue to struggle with issues including mental health, criminal and sexual exploitation, poverty and homelessness.

“While that means ensuring local organisations including councils have the additional money they need to offer this support, a coordinated plan across government departments is also required.”

The children’s commissioner for England, Anne Longfield, said: “Older teenagers often fall through the gaps in services because they are not being treated as children but still need the right support to help them as they grow into adulthood.

“I am particularly concerned about children who are invisible to services – those not meeting the thresholds for state support – but who without the right help could end up in crisis, without a good education or employment, at risk of homelessness or getting involved in gangs,” she added.

Cat Smith,the shadow minister for youth affairs, said: “It’s time for the government to take action; I urge ministers to set up this taskforce without delay.”

The mental health and inequalities minister, Jackie Doyle-Price, said: “Providing the right mental health support to children and young people is a key priority for this government – this includes making sure they can access the help they need during the challenging transition to adulthood.

“We know that classing children’s health services to 18 and adult services thereafter does not always make for the best care for young people. So I will ensure 70,000 more children have access to mental healthcare,” she added.