Former Deputy Prime Minister Lord Heseltine has urged the Lords not to "create a constitutional crisis" by blocking planned cuts to tax credits in a vote on Monday.
The senior Conservative, who served in both the Thatcher and Major governments, was speaking to ITV News amid a vote in the House of Lords on the measures on Monday.
The unelected chamber usually does not change finance laws, but a rarely-used "fatal motion" has been threatened by the Liberal Democrats, which could see the measures scrapped.
Lord Heseltine also called on George Osborne to "keep his nerve" over the measures, which opponents say could cost millions of households up to £1,300 a year.
"Nobody knows how genuine these figures are," he said of the estimates, noting that families' financial circumstances will be affected by a range of other factors including "compensatory" benefits and increasing employment.
They are not there to interfere with the settled will of the House of Commons, warned Boris Johnson.
A working mum who would be hit by Tax Credit cuts spoke to ITV News ahead of the House of Lords vote on the changes.
Peers will have to decide whether to delay changes to the tax credits bill, review them or scrap it altogether.