The Welfare Reform Bill was announced in the Queen's Speech
The government's plans to reform the UK welfare system have come under criticism for being "misplaced and out of time".
The Welfare Reform Bill aims to give incentives for people to move from benefits into "sustained employment".
But Paul Kenny of the GMB union said there was "no point" in "putting people into non-existent jobs".
Work and Pensions Secretary James Purnell said the system had to be fair to taxpayers and those needing support.
Mr Kenny also called for the measure to be dropped.
The bill aims to tackle unemployment and increase the responsibility of those on benefits to return to work.
Ministers said the bill would help achieve the goal of an 80% employment rate, which they said would be the highest of any major industrialised country.
The government also says the legislation will help 300,000 more lone parents and a million more older people into work.
Under the bill, Income Support will be abolished and claimants moved onto either Jobseeker's Allowance or Employment and Support Allowance if they are ill.
A system of benefit sanctions will be introduced for people who refuse to attend job centres.
Partners of benefit claimants will also have to look for a job, and there will be work-focused interviews for the over-60s who wish to be employed until they reach retirement age.
But Steve Ford, chief executive of the Parkinson's Disease Society, said: "Nearly half of people of working age with Parkinson's are on incapacity benefits.
"Parkinson's disease is a complex and fluctuating condition, with the potential for someone affected to appear healthy one minute, but to be incapacitated the next.
"We are concerned that the new work capability assessments have the potential to deem someone with Parkinson's capable of seeking work when they are not, and being forced to go through work-seeking activities under threat of sanctions when they are not well enough."
TUC general secretary Brendan Barber said the welfare reform proposals appeared to be a "leftover from pre-recessionary times".
He added: "They are based on the view that anyone can get a job if they have the right attitudes, training and advice.
"Of course we should not tolerate those who cheat the system, but with the dole queue set to grow by more than a million, the newly redundant need help, guidance and decent benefits, not to be treated as potential benefit scroungers.
"Ordinary people need to know the government is on their side in these difficult times."
Gordon Lishman, director general of Age Concern, said it supported efforts to get over-50s back into work.
"But this will only work if jobs are available and the right kind of support is on offer," he added.
"This is not currently the case and sanctions to withdraw benefits should not be introduced until they are."
Mr Purnell said the government wanted to help people "build the skills and confidence they need to move off benefits and into work".
He said: "It is crucial that we do everything we can to make sure people stay connected to the labour market during tough times, and that we don't repeat the mistakes of the 80s and 90s when thousands were written off and consigned to a life of benefit dependency.
"The system must be fair - both to people that need support, but also to the taxpayer.
"It is absolutely right that people should be expected to take reasonable steps to prepare themselves for, or get back to, work if they are able to.
"We know that our support changes the lives of families and individuals and the Welfare Reform Bill will increase the expectation to take up the support on offer for those who can."
Liberal Democrat work and pensions spokesman Jenny Willott said the new bill was a "frank admission that after 11 years, this government still has a mountain to climb on welfare reform".