The government's controversial decision to cut tax credits has come under renewed pressure as Lord Newby said the House of Lords will obstruct the legislation, and Prime Minister David Cameron would face a "constitutional crisis of his own making" if he took measures against the Lords in response.
The government no longer has a majority in the House of Lords as over 100 Liberal Democrat peers turned to opposition after the Conservatives won the General Election, meaning the government faces real opposition from the upper house.
However, there is a long running convention that means the Lords should not hold up financial measures, even though in theory it can.
Opposition peers want to vote the plans down with what's been termed a "fatal motion". Newby, who is Lib Dem chief whip in the Lords, said the government is not permitting enough discussion in either House on legislation which is unamendable. Speaking to BBC Radio 4's Today programme he said:
The measures which the Chartered Institute of Taxation - not a bunch of raving lefties - say would be devastating for the majority of tax credit claimants.
We believe parliament should have more opportunity to debate these issues and the government needs to think again about them.
But if the Lords do hold up the legislation, Cameron could then flood the Lords with Tory peers.
Newby said the Prime Minister is acting like a "school boy bully. He's being challenged in the playground so he's threatening to bring round lots of his mates to duff us up. He would be creating a constitutional crisis, but that would be of his making, not ours".
However, Lord Butler hit back, saying a fatal motion would amount to the Lords getting too big for its boots. "The fact is the House of Commons has passed the legislation and the Lords has long accepted the supremacy that the Commons decides tax and expenditure measures. This is clearly a tax measure and central to the government's cut of £12bn in welfare, on which they were elected so recently."
Chancellor George Osborne had previously come under fire from the House of Commons, including MPs from his own party, who were frustrated over the changes, which mean three million families will lose on average £1,000 a year, according to research from the IFS.