AFAIK what's proposed is a blanket ban. Here's an open letter to Cameron from many UK charities (published in the Times).
Sir,
The Prime Minister?s proposals to cut benefits for under-25s will have a disastrous impact on thousands of young adults, especially at a time of record youth unemployment.
Tens of thousands of young people simply don?t have stable family and friends to go back to if they lose their job or fall ill. Young mums and dads, people whose parents have downsized or those who have experienced family breakdown will be particularly affected.
You don?t root out any excesses in the system by abolishing an entire safety net for young people. They too pay into the state safety net when they are working in the belief that it will be there for them when they need it. Evidence shows that when young adults claim support, many will need it for less than six months before they get back on their feet and into work.
We welcome open debate about the challenge of giving young people a fair start and making work pay. But with these ill-conceived proposals, the Prime Minister has gone too far.
Yours,
Anne Marie Carrie, Chief Executive, Barnardo?s
Seyi Obakin, Chief Executive, Centrepoint
Matthew Reed, Chief Executive, The Children's Society
Leslie Morphy OBE, Chief Executive, Crisis
Jane Slowey, Chief Executive, The Foyer Federation
Fiona Weir, Chief Executive, Gingerbread
Matt Harrison, Interim Chief Executive, Homeless Link
Campbell Robb, Chief Executive, Shelter
Jean Templeton, Chief Executive, St Basils
Ian Green, Chief Executive, YMCA England