The government is headed for a bruising defeat on Monday over tax credits when the measure is voted on in the House of Lords. A cross-party alliance of peers - from the Tory backbenches, the Labour party, the Liberal Democrats, and peers of no party at all - are all but certain to vote down the measure when it comes before the House of Lords.

Usually, the unelected Lords cannot vote down finance measures - but the Treasury's decision to put the change through not as a financial bill but as a statutory instrument means the House of Lords can, on this occasion, vote down the measure.

The government is doing its utmost to avert the defeat. Baroness Meacher, a crossbench peer - peers who do not take the whip of any party in the House of Lords - came under pressure to withdraw her motion killing off the measure entirely, while Downing Street sources are briefing that defeating the cuts would lead to a mass creation of Conservative peers to rebalance the political composition of the Upper House. No government has "packed" the Lords in this way since 1711, when Queen Anne created nine new peers in order to create a majority for the treaty to end the War of the Spanish Succession.The threat of it alone was enough to force the Lords to vote through a bill limiting its power in 1911.

The government's argument is that the Salisbury-Addison Convention - the deal brokered between Lord Salisbury, leader of the Conservatives in the Lords, and Lord Addison, leader of the Lords during the Labour government of 1945-51 - under which the House of Lords votes through all manifesto commitments means that the Upper House has no choice but to approve the measure. But there is a widespread view in the Lords that far from being a manifesto commitment, Cameron's pre-election promise on tax credits means that they are more than entitled to vote it down.

But it may be that the House of Lords are walking into a trap. The use of a statutory instrument, rather than a finance bill, has opened the government up to a defeat - but may give the government a pretext to reassert the longstanding Conservative primacy in the Upper House. Throughout the 1979-97 Conservative government, Tory peers were the largest single bloc of peers, and it wasn't until the final year of Tony Blair's government that the Labour group eclipsed the Conservative one. The Tories now have the single largest group of peers again, but they are some way off their historic dominance of the Upper House. Labour aides are dismissive of this theory however - they think it is more likely that the Treasury simply forgot that a statutory instrument could be used in this way. But it could be that defeat on Monday paves the way for a reassertion of Tory dominance in the weeks to come.