The coalition begins its toughest test yet, as MPs begin discussing House of Lords reform with Tory rebels confident of giving the Government its first major Commons defeat.

Deputy Prime Minister Nick Clegg will appeal for unity around proposals for an 80 per cent elected upper chamber, as he starts what promises to be a fiery two-day debate on the major constitutional change.

It will end on Tuesday in a vote that a senior Liberal Democrat source warned would be a "highly significant moment" for the two-year-old power-sharing administration.

While the reforms themselves are expected to clear their first parliamentary hurdle with Labour support, up to 100 Tories are said to be ready to join with the Opposition to throw out the timetable for further stages.

The legislation, central to the Lib Dems' agenda in the coalition, would slim down membership from 800 to 450 and complete the removal of hereditary peers, replacing them with members elected under a form of proportional representation for a single 15-year term.

Another 90 members will be appointed by a statutory Appointments Commission on a non-party basis and there will also be 12 Church of England bishops, down from 26 church representatives.

Former Commons Speaker Baroness Boothroyd said: "I have been in Parliament nearly 40 years, I have never witnessed such a reckless attempt to change the British constitution, and to do it on such spurious grounds, quite frankly.

"What they are seeking to do is limit debate on this major constitutional issue, I believe it is an outrage, and I think it is an abuse of Parliament."

Lady Boothroyd insisted she was in favour of reform, but these proposals could lead to clashes between the chambers over issues such as war.

Constitutional Reform Minister Mark Harper defended the Government's attempt to limit the time for debate, insisting the plans had been extensively scrutinised.

The Tory minister said: "I think colleagues will support it when they look at these proposals, they are well thought through, they have been scrutinised by a committee of both Houses."