Image caption Lord Ashdown is threatening to vote against the coalition for the first time

Former Liberal Democrat leader Lord Ashdown has said he will vote against coalition plans for a cap on the total benefits paid to a single household.

The Lib Dem peer said he could not back the plan for a £26,000 annual limit in a vote on Monday without measures to cushion the impact on those affected.

Deputy Prime Minister Nick Clegg said a limit was justified but he would look at "transitional arrangements".

Critics have urged a rethink, including exempting child benefit from the cap.

But Work and Pensions Secretary Iain Duncan Smith has urged those opposed to aspects of the cap, including leading bishops, to think of those who pay taxes while some unemployed people live in large houses at public expense.

'Unacceptable'

In an interview with the Sunday Times, Mr Duncan Smith admitted his plans for a cap on working-age benefits of £500 a week or £26,000 a year - equivalent to the average wage earned by working households after tax - could face defeat in the Lords on Monday.

WHAT CAP WILL INVOLVE From April 2013, the total amount of benefit that working-age people can receive will be capped so that households on out of work benefits will no longer receive more than the average weekly wage earned by working households.

The cap will apply to the combined income from the main out-of-work benefits - Jobseeker's Allowance, Income Support, and Employment Support Allowance - and other benefits such as Housing Benefit, Child Benefit and Child Tax Credit, Industrial Injuries Disablement Benefit Q&A: Welfare changes row

Lord Ashdown has become the most senior figure to say he is opposed to the plan as currently drafted, telling Sky News that the proposals as they stood were "completely unacceptable".

"I have voted with the government on everything until now," he said.

"But this legislation, in its present form, I cannot accept."

He suggested senior Lib Dems were pushing "very hard" for measures to help those most dependent on benefits to cope with the change and prevent them falling into greater hardship.

'Simple principle'

Earlier, Mr Clegg told the BBC he was willing to look at how the changes were implemented but he "completely backed" Mr Duncan Smith on the principle of the cap.

"Of course we need to look at transitional arrangements and Iain Duncan Smith has made it quite clear that we need to do that," he told the Andrew Marr Show.

Media playback is unsupported on your device Media caption Nick Clegg outlines details of the benefits cap

Things that could be looked at, he added, included "the place of children who were born, if you like, innocently into another set of rules".

But he added: "The basic principle that that cap should be £500 [per week] so that you can't on benefits earn more than if you went out and worked, I think that's got to be a simple principle that most people would subscribe to."

Mr Duncan Smith has acknowledged the result of the Lords' vote could depend on independent crossbenchers, including leading bishops who have criticised the plans.

He told the Sunday Times: "The question I'd ask these bishops is, over all these years, why have they sat back and watched people being placed in houses they cannot afford? It's not a kindness.

"I would like to see their concerns about ordinary people, who are working hard, paying their tax and commuting long hours, who don't have as much money as they would otherwise because they're paying tax for all of this."

Child benefit

The Rt Rev John Packer, Bishop of Ripon and Leeds, said he was not opposed to all aspects of the bill but believed child benefit should be exempted from the cap.

"What we're talking about tomorrow is children in families where the welfare benefits have been cut to a point where they are less than Parliament actually has said they should be, because that's what a cap does," he told the BBC's Sunday Morning programme.

Labour said the £26,000 figure used to calculate the cap was not "entirely accurate" and those on welfare would now be getting less than the equivalent annual wage.

"In principle we are not opposed to a benefit cap," shadow business secretary Chuka Umunna told the BBC's The Sunday Politics. "What we are opposed to is the way the government has done it."

Peers have already inflicted a series of defeats on the government's flagship Welfare Reform Bill, which applies to England, Scotland and Wales, but ministers say they are determined to get key changes through Parliament.