New measures aimed at the long-term unemployed come into force on Monday with people facing benefit sanctions if they refuse to take part.
The Government said Jobcentre staff will have more options to support the hardest to help under Help to Work.
There will be "intensive" coaching, a requirement to meet an adviser every day or doing community work for up to six months.
Unite urged the bosses of charities not to take part in the programme, describing it as "workfare."
Ministers pointed out there are more than 600,000 vacancies in the economy at any one time, saying that the new measures were intended to help unemployed people fill them.
The voluntary work could include gardening projects, running community cafes or restoring historical sites and war memorials. The placements will be for up to six months for 30 hours a week and will be backed up by at least four hours of supported job searching each week.
Prime Minister David Cameron said: "A key part of our long-term economic plan is to move to full employment, making sure that everyone who can work is in work.
"We are seeing record levels of employment in Britain, as more and more people find a job, but we need to look at those who are persistently stuck on benefits.
"This scheme will provide more help than ever before, getting people into work and on the road to a more secure future."
Work and Pensions Secretary Iain Duncan Smith said: "Everyone with the ability to work should be given the support and opportunity to do so.
"The previous system wrote too many people off, which was a huge waste of potential for those individuals as well as for their families and the country as a whole. We are now seeing record numbers of people in jobs and the largest fall in long-term unemployment since 1998.
"But there's always more to do, which is why we are introducing this new scheme to provide additional support to the very small minority of claimants who have been unemployed for a number of years.
"In this way we will ensure that they too can benefit from the improving jobs market and the growing economy."
Unite assistant general secretary Steve Turner said: "This scheme is nothing more than forced unpaid labour and there is no evidence that these workfare programmes get people into paid work in the long-term.
"We are against this scheme wherever ministers want to implement it - in the private sector, local government and in the voluntary sector.
"The Government sees cash-starved charities as 'a soft target' for such an obscene scheme, so we are asking charity bosses to say 'no' to taking part in this programme. This is a warping of the true spirit of volunteering and will force the public to look differently at charities with which they were once proud to be associated.
"It is outrageous that the Government is trying to stigmatise job seekers by making them work for nothing, otherwise they will have their benefits docked."
Shadow employment minister Stephen Timms said: "Under David Cameron's government nearly one in ten people claiming Jobseeker's Allowance lack basic literacy skills and many more are unable to do simple maths or send an email. Yet this Government allows jobseekers to spend up to three years claiming benefits before they get literacy and numeracy training.
"A Labour government will introduce a Basic Skills Test to assess all new claimants for Jobseeker's Allowance within six weeks of claiming benefits.
"Those who don't have the skills they need for a job will have to take up training alongside their job search or lose their benefits. Labour's Basic Skills Test and our Compulsory Jobs Guarantee will give the unemployed a better chance of finding a job and will help us to earn our way out of the cost-of-living crisis."