Some of the government's New Deal programmes are becoming "less successful" at getting the long-term unemployed into work, MPs have said.
MPs said job entry rates on some schemes were declining
The Commons public accounts committee urged ministers to make schemes more flexible to increase take-up.
It said 1.6 million more economically inactive people - many of whom have not worked since age 16 - had to be brought into employment.
The government said it was bringing in reforms to give "further support".
Ministers say they want to get 80% of working-age adults into employment.
The committee said the current rate of 74.9% was among the highest in countries comparable to the UK.
But job entry rates for the New Deal for Lone Parents, New Deal 25 Plus and the New Deal for Young People had been "declining or stable for some years".
The committee's report said: "One possible factor is that the composition of the client group has been altered by both the initial success of the programmes and the high overall employment rate.
"As people with relatively few barriers to work move into employment, those remaining on benefits and within the New Deal client group face greater barriers and are therefore harder to help into work."
The MPs said there were three million households in the UK where no-one over the age of 16 had a job, partly caused by "barriers to work including skills deficits, disability and caring responsibilities".
The committee recommended "early intervention and targeted support"
Around 60% of workless households were concentrated in 40 districts of the UK, but services such as training and advice were not targeted enough at these areas.
'Hard to reach'
The committee's chairman, Conservative MP Edward Leigh, said: "The evidence is that many New Deal programmes are becoming less successful at finding work for their clients, perhaps because the hardest to help are becoming an increasingly large proportion of those clients."
Liberal Democrat work and pensions spokesman Danny Alexander said: "This report highlights problems that have been evident for a long-time - the government's one-size-fits-all programmes are ignoring hard to reach groups that must be supported back into work."
Employment Minister Stephen Timms said: "We have a record number of people in work in the UK and since 1997 have cut the number of workless households by almost 200,000.
"Our welfare reforms will provide greater support to help those furthest from the workplace into employment.
"We are introducing a flexible New Deal with a more personalised approach, enabling us to tailor the support available to individual needs and address the specific barriers preventing individuals from working.
"Our £1.5bn Working Neighbourhoods Fund is all about targeting money at our most disadvantaged communities, giving them the ability to decide how they tackle the problems facing their area."