Labour would also create a "youth allowance" for low-skilled college students, who do not now receive help.
The measures, which will affect young people with very low literacy and numeracy skills, are designed to fix what Labour insist is a broken system.
But critics will question whether the party's proposals amount to the state changing the point at which people are considered adults.
Mr Miliband will also reiterate plans to increase Jobseeker's Allowance paid to those who have contributed National Insurance payments for a long time.
Under Labour's proposals, young people aged 18 to 21 who lack the skills to get a decent job would no longer be entitled to out-of-work benefits.
Instead, their Jobseekers' Allowance would be replaced with a means-tested payment, conditional upon them being in training and linked to their parents' wealth. The move would offer support to youngsters who do more than 16 hours a week training or further education but who are not at university and who currently receive nothing.
A Labour source said: "We believe this will strengthen the college system and make sure that people are getting the support they need."
Exemptions would be made for those who already have qualifications, for young parents and for those with disabilities.
In his speech, Mr Miliband will insist that the changes will be "progressive not punitive". He will say: "Britain's young people who don't have the skills they need for work should be in training not on benefits."
The speech comes after a poll by Ipsos MORI yesterday showed that 49 per cent thought Mr Miliband should be replaced as Labour leader before the General Election next year.
Last night Conservative Party chairman Grant Shapps said Mr Miliband's plans were "a recipe for more spending on welfare, more borrowing - and more taxes to pay for it."