Teaching Comparative

Labour and the Lib Dems have defended their challenge to tax credit cuts as ministers warned of taking an "unprecedented constitutional path".

The House of Lords will vote on motions that could delay the controversial cuts or scupper them altogether.

Ministers say peers do not have the right to block financial measures approved by the House of Commons.

But Labour's shadow welfare secretary Owen Smith said such arguments were a "total sideshow"... 

Tax credits are a series of benefits introduced by the last Labour government to help low-paid families. There are two types: Working Tax Credit (WTC) for those in work, and Child Tax Credit (CTC) for those with children.

Under government proposals, the income threshold for Working Tax Credits - £6,420 - will be cut to £3,850 a year from April.

In other words, as soon as someone earns £3,850, they will see their payments reduced. The income threshold for those only claiming CTCs will be cut from £16,105 to £12,125... 

There will be similar reductions for those who claim work allowances under the new Universal Credit...

Opponents of the tax credit changes say they will leave millions of existing recipients - many of whom work but are on low incomes - some £1,300 a year worse off when they come into effect in April.

But ministers say that taking into account other changes, such as the introduction of the new national living wage, further increases in the personal tax allowance and an extension of free childcare, the majority of existing claimants will be better off.

The government is more vulnerable to defeats in the House of Lords, where it has no majority.

Lib Dem peer Baroness Kramer said the idea of a "constitutional bar" was "complete and utter rubbish".

She claimed the Conservatives were trying to make the issue a constitutional one because they have "lost the argument".

The measures have been approved on three occasions by the Commons since June but there has been growing unease on the Conservative benches about their impact and calls for Chancellor George Osborne to rethink his approach...

When they consider the proposals later, peers could be asked to vote on as many as four motions urging a different approach. Most attention has focused on a motion tabled by Liberal Democrat peer Baroness Manzoor "declining to approve" the proposals.

Constitution wars: It should not really be much of a problem - the House of Lords is not traditionally supposed to block financial legislation that has the backing of MPs.

This principle was established in 1911 during the constitutional gridlock that followed a decision by peers to block the Liberal Party's "people's budget".

But nothing is ever cut and dried in Britain's fluid, unwritten constitution. And both sides are angrily trading precedents and claiming that their opponents are overstepping the mark. If they could only agree where the mark is.

Ahead of Monday's proceedings, there has been debate over whether the House of Lords has the authority to oppose the changes.

The Upper House, whose main function is as a revising chamber, has no powers to amend or block government money bills but the tax credit changes are incorporated in a so-called statutory instrument rather than primary legislation.

According to parliamentary records, peers have killed off secondary or delegated legislation supported by the Commons on five occasions since 1945: in 1968, 2000 (twice), 2007 and 2012...