The In Defence of Youth Work campaign group said there is an urgent need for government to commit to "all-year locally-based youth work facilities for young people who choose to engage".
It said that, in light of the recruitment target for the National Citizen Service being reduced by nearly a third, the equivalent portion of its £1.26bn budget - equivalent to around £420m - could be reallocated to open youth work - taking place in youth clubs, youth projects, youth centres, or on the street.
A discussion paper for the campaign group, authored by youth work consultant and academic Bernard Davies, also suggests that the NCS could consider making open youth work "core practice".
"With large swathes already going, local authority youth services may have disappeared altogether by 2020," the paper states.
"Open youth work is increasingly dependent on a few community-based, minimally funded voluntary organisations heavily reliant on philanthropic judgments on who is deserving.
"Relatively well-resourced regional and national bodies take on top-down government and business contracts, thereby putting at risk their historic ‘voluntary' role of challenging those in power and standing with those marginalised from decision-making.
"Urgently needed are now explicit state commitments to all-year locally-based youth work facilities for young people who choose to engage."
Earlier this year the NCS announced a three-year partnership deal which will see the Scout Association "integrate" the NCS experience within Scouting in order to "support the growth of both organisations and offer new opportunities for social mixing".
The NCS first launched in 2011, initially to a few thousand young people, but provision has been expanding each year, with the government setting aside around £1.2bn to deliver NCS up to 2020.
However, NCS participation has struggled to keep pace with targets, and, in February, the Department for Culture, Media and Sport revised its aim to grow NCS participation by 2020/21 from 360,000, which had been in the Cabinet Office's single departmental plan, down to 247,000.