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Peers are to launch a bid to block a welfare reform which they claim could cause hundreds of thousands of families to lose an average of more than £1,000 a year.

Liberal Democrat leader Tim Farron branded the changes to universal credit as “tax credits mark two” as he threatened to force a vote to scupper them.

He claimed the plans would mean that if a single parent found a new partner it would cost them an average of £1,080 a year.

Analysis by the Lib-Dems suggested that up to half a million claimants could be affected, based on figures showing that around half of single parents find a new partner within five years of breaking up with a previous partner.

Last year, peers forced George Osborne to ditch a shake-up of tax credits which experts warned would have hit millions of families.

The reform aimed to save £4.4 billion from the welfare bill but the House of Lords voted 289 to 272 to delay the changes for three years until measures could be introduced to protect poor families.

Condemning the universal credit changes, which are due to come into effect next year, Mr Farron said: “This is tax credits mark two. We stopped that and we will stop this.

"The Conservatives claim they want to support two-parent families, yet their attack on universal credit will mean that single parents who find a new partner will actually be penalised.”

However, the Department for Work and Pensions said the Lib-Dem claims were “scaremongering” and failed to recognise additional support that was not offered under the old system.

The reforms are due to be debated in the Lords tomorrow and Baroness Manzoor, the Lib-Dem work and pensions spokeswoman, has tabled a motion to repeal the regulations bringing in the universal credit changes.

With just eight MPs, Mr Farron’s party has been left very weak in the Commons but it has used its peers and the lack of a Tory majority in the Lords to challenge the Government over welfare and other issues.

Iain Duncan Smith, the Work and Pensions Secretary, brought in universal credit.

It combines a series of benefits and its main aim is to ensure that work pays, as well as simplifying the welfare system. Given its complexity, it was decided to gradually roll it out across the country.

A DWP spokesman said: “This kind of scaremongering completely fails to recognise those who gain significantly under universal credit and the fact that claimants are moving into work faster and earning more than under the old system.”@nicholascecil