Updated

Government welfare reforms that include a contentious cut dubbed the "bedroom tax" will cause upheaval for some of Britain's most vulnerable people, religious leaders and anti-poverty activists claim.

The measure, which takes effect Monday, will reduce rent subsidies to social housing tenants if they have a spare bedroom.

The government — which prefers the term "under-occupancy penalty" — says it is one of a series of changes that will make the country's unwieldy welfare system simpler, cheaper and fairer.

But thousands of trade unionists, advocates for the disabled and anti-poverty campaigners held protest marches against the change on Saturday, and on Sunday four churches released a joint criticism of the reforms. The Baptist Union of Great Britain, the Methodist and United Reform churches and the Church of Scotland argued that "the cuts are unjust and that the most vulnerable will pay a disproportionate price."

"Our feeling is that these benefit changes are a symptom of an understanding of people in poverty in the United Kingdom that is just wrong," Methodist spokesman Paul Morrison told the BBC. "It is an understanding of people that they somehow deserve their poverty, that they are somehow 'lesser', that they are not valued."

Archbishop of Canterbury Justin Welby, leader of the Anglican church, has also criticized the welfare reforms.

The British government is trying to reduce public spending by 50 billion pounds ($76 billion) by 2015 in a bid to deflate Britain's ballooning deficit and kick-start its spluttering economy. It says its welfare reforms will save 4.5 billion pounds by 2014-15.

The measures include changes to disability benefits, below-inflation increases and, eventually, the replacement of a patchwork of housing, unemployment and parental benefits with one payment called the Universal Credit.

The Department for Work and Pensions says the spare-bedroom levy — a cut of 14 percent to households with one extra room and 25 percent for two — will save taxpayers money and will help free up social housing for families because people with too many rooms will downsize.

"It is wrong to leave people out in the cold with effectively no roof over their heads because the taxpayer is paying for rooms which aren't in use," Conservative lawmaker Grant Shapps told Sky News.

Officials say the new rules won't apply to retirees, or to those who really need extra space, such as parents of severely disabled children.

But campaigners say the "bedroom tax" has already produced injustices. Parents whose children are not considered disabled enough by local officials have been told they must pay. So has a bereaved couple who couldn't bear to change the bedroom of their 7-year-old daughter after she died of brain cancer.

To its opponents, the "bedroom tax" is an indignity on a par with the "poll tax," a levy on every adult that sparked violent protests and helped bring down Prime Minister Margaret Thatcher in 1990. Her successor, John Major, scrapped it.

The government says its welfare reforms are modest measures that will encourage people to get off welfare and find jobs. In tough times, officials say, everyone must make sacrifices.

Opponents ask why the government can't tax mansions or second homes, rather than the poor. And they allege the cuts will force impoverished residents to move from homes and neighborhoods where they have lived for years.

Frank Field, a minister in the previous Labour administration and now a government adviser on fighting poverty, told The Guardian newspaper that "the government is introducing social and physical engineering that Stalin would have been proud of."

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Jill Lawless can be reached at http://Twitter.com/JillLawless