Later today the welfare bill will be debated in the Lords – with Labour, Lib Dem and crossbenchers set to oppose or propose amendments to current plans.

Under the proposals put forward by the government, there would be a £26,000 cap on overall benefit payments for a household.

But the Labour line on Iain Duncan Smith's welfare reform plan has been surprisingly mild.

As one Labour MP told me: "We can't oppose the benefits on the grounds of fairness. We can oppose how they are implemented on the grounds of fairness. "

They claimed it was also a question of economic credibility. "We can't launch a blanket objection to something that is proven to have a substantial cost to it, is perceived to be unfair and seems to have some element of public support."

A YouGov poll suggests that 76% of the public are in favour of the benefits cap, including 69% of Labour voters.

There is also something to the idea that a quieter objection avoids giving the government ammunition to use against Labour. As James Forsyth at The Spectator notes: "Chris Grayling has already declared that tonight’s vote in the Lords is ‘a test of Ed Miliband’s leadership’."

Shadow employment minister Stephen Timms seemed to be treading a cautious path on the Today programme this morning.

He said: "We support the idea of a benefit cap but we are worried that the government wants to do it in a very damaging way that is likely to end up costing more than it saves."

Kiran Stacey has also picked up on Labour's gentle opposition. Over on the FT blog, he believes members of the Lords are being more robust: "Although peers are sticking to the official line – that there should be a benefit cap but not in its current form – shadow ministers are worried that by picking a very visible fight over such a popular measure they are alienating voters."

The government has maintained that the changes are not intended to punish benefit claimants.

A No10 spokesperson this morning added that the proposals are "about ensuring people on benefits have to make the same choices as working families".

Tags: Benefits cap, Iain Duncan Smith, Stephen Timms