BBC News website age & disability correspondent
Organisations representing disabled and older people say reforms of incapacity benefit need to be properly funded.
The government hopes to get 1m people back to work
Most say that without the support that has been given to people on the Pathways to Work scheme, the changes are unlikely to succeed.
There are also concerns that incapacity benefit claimants are being blamed for a deep-rooted social problem.
They say employers must rid themselves of their preconceptions about giving jobs to older and disabled people.
The Third Age Employment Network (TAEN) - which supports older people in the job market - says the reforms set out in the government's green paper are a vital part of the response to demographic change and increased life expectancy.
"We can no longer use incapacity benefit as a surrogate form of early retirement," said TAEN chief executive, Patrick Grattan.
He described the initial trials of Pathways to Work as having been excellently prepared and delivered.
But he thinks that government spending plans will not stretch to a nationwide roll out of the scheme.
The reforms are "a step in the right direction", according to the Disability Rights Commission (DRC) - the body that oversees the rights of disabled people in the UK.
But chairman Bert Massie says he wants to see more detail in order to be sure that the proposals really will improve opportunities for disabled people.
The DRC is concerned to ensure that contracts awarded to private and voluntary sector bodies will deliver a high quality service in line with best practice on disability equality in the public sector.
"As detailed plans are made, we urge the government to undertake a proper scrutiny of these proposals with the DRC," said Mr Massie.
The Disability Benefits Consortium (DBC) - a broad-based coalition that includes age as well as disability groups - has described the proposals as a "missed opportunity".
The group believes that a lack of additional resources, the use of sanctions and the emergence of a divide between those who can and cannot work will undermine the system.
The DBC also says that there is a lack of opportunities for people with learning disabilities and mental health conditions.
For its part, the Shaw Trust - a charity that helps disabled and other socially excluded groups to gain employment - has welcomed the increased role proposed for the voluntary sector.
"We hope that we will at last have a real chance to help the 1.2 million people who are currently economically inactive but who want to work," said chief executive Ian Charlesworth.
But he warned that the government should provide resources if it is serious about helping more people to find and retain work.
"These proposals will only be effective if they are backed up with the right support."
Campaigning charity Scope has issued its own, alternative green paper on incapacity benefit, and warns that disabled people risk being penalised if the government's proposals are implemented.
Incapacity Benefit should not be means tested, Scope thinks, and there should be no obligation for people approaching retirement age to have to find work.
The charity's executive director for diversity, politics and planning, Andy Rickell, says the government still sees the barriers to work as being situated with the incapacity benefit claimants themselves.
"They appear to have ignored the fact that, despite the Disability Discrimination Act, there's still widespread discrimination against disabled people in the job market," he said.
Scope's views have been echoed by the DBC.
"Instead of threatening sick and disabled people with benefit sanctions, the government should be concentrating its efforts on developing an effective retention strategy which would prevent people being pushed out of the labour market in the first place," said DBC spokeswoman, Lorna Reith.
Mental health concerns
Mental health charity, Mind, is worried that people could be forced to return to work too soon - without proper support both in and out of the workplace.
And it says that medical assessments made by staff who are not properly trained to deal with mental health could result in bad decisions being made.
A recent survey of employers carried out by Mind showed that fewer than 10% of companies had a mental health policy.
"The majority of people with mental health problems want to return to employment as soon as possible," said policy director Sophie Corlett.
"But they need to be helped and supported, not goaded and ultimately forced to return to work before they are ready to do so."
Ms Corlett believes that the threat of sanctions could put people with mental health conditions under increased and unnecessary pressure.
Those groups with concerns about how the reforms will be implemented will now be pressing the DWP for changes during the consultation period.