A coalition of leading churches has accused the government of perpetuating myths about poverty in an attempt to justify dangerous welfare cuts.

In a joint report, the Baptist Union of Great Britain, the Methodist church, the United Reformed church and the Church of Scotland said that the "systematic misrepresentation of the poorest in society" was a matter of injustice that all Christians should challenge.

The churches spoke out before the introduction of the so-called "bedroom tax" on Monday, one of several major welfare cuts that Labour claim will collectively cost the average family almost £900 a year when previous cuts introduced by the coalition are included.

Paul Morrison, public issues policy adviser at the Methodist church, said the churches were concerned that the benefit cuts were "a symptom of an understanding of people in poverty in the United Kingdom that is just wrong".

Speaking to the BBC, Morrison said: "It is an understanding of people that they somehow deserve their poverty, that they are somehow 'lesser', that they are not valued. The churches believe that they are valued and we believe that they should be treated much more fairly than they are being."

In their report (pdf), which was published earlier this year, the four churches accused politicians and the media of promoting six myths about the poor: that they are lazy; that they are addicted to drink or drugs; that they are not really poor; that they cheat the system; that they have an easy life; and that they caused the deficit.

Morrison explained: "We saw that people who we value, who we believe God values and God loves, we saw them being insulted day in and day out in the media, and that needed to stop. The consequence of the attitudes towards the poor is that welfare cuts like this become more acceptable, so it's right that we criticise that too."

Morrison spoke out as Grant Shapps, the Conservative party chairman, used an interview with the Sunday Telegraph to highlight figures apparently showing 878,300 people who were on incapacity benefit dropped their claims rather than complete a medical assessment.

That was a third of the total, he said, and included people on benefits because of blisters, acne, and strains and sprains. Of the 1.44m who have been reassessed, 55% were found fit to work immediately and another 23.9% able to do some level of work, leaving just 232,000 declared not fit to work.

"Welfare makes up a third of this country's spending, so it's our job to make sure it's getting to the people who really need it," Shapps said.

In a separate interview, Steve Webb, the Lib Dem welfare minister, told Radio 5 Live on Sunday that while the churches were criticising the government for cutting benefits by too much, other commentators were accusing the government of not going far enough.

"On the one hand we've got the churches accusing us of savage cuts, and on the other hand the rightwing papers saying: 'What? They haven't gone far enough. They're only slowing the rate of growth of spending'. I think that gives you a bit of context," Webb said.

He said measures such as universal credit showed the government was reforming the system, not just cutting benefits. And he accused the churches of not accepting the need to make savings.

"There's nothing in what the churches are saying – and I speak as a member of my local church – there's nothing in what they're saying that helps us with the battle to we've got in getting the nation's finances in balance," Webb said.

In a separate move, the "bedroom tax" came under fresh criticism on Sunday when David Orr, the chief executive of the National Housing Federation, told the Observer that the tax would harm the lives of hundreds of thousands of people. It comes into force this week alongside a range of other tax and benefit changes.

"The bedroom tax is one of these once-in-a-generation decisions that is wrong in every respect," he said. "It's bad policy, it's bad economics, it's bad for hundreds of thousands of ordinary people whose lives will be made difficult for no benefit – and I think it's about to become profoundly bad politics."