The coalition begins its sternest test yet as MPs begin discussing House of Lords reform with rebels confident of inflicting the government's first major Commons defeat, with 70 Tories putting their name to a letter opposing the plans.

Friction between the coalition partners is building as 100 Conservative MPs threaten to vote against Liberal Democrat plans to make the House of Lords 80%-elected.

Deputy prime minister Nick Clegg will appeal for unity around proposals for an elected upper chamber as he opens what promises to be a fiery two-day debate on the major constitutional change.

It will culminate 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.

Losing the programme motion would leave the legislation vulnerable to being "talked out" by opponents and leave the government in what the Lib Dem source said would be "uncharted territory".

"That's going to be a very serious situation for the coalition", they warned - pointing out that the reform had been promised in all three main parties' manifestos and was part of the coalition agreement.

On Monday morning a letter signed by 70 Tories revealed the scale of the challenge faced by the government to get its preferred timetable though.

The letter sent to other seen by the BBC calls for "full and unrestricted scrutiny" of the Bill.

Liberal Democrat Foreign Office Minister Jeremy Browne said Tories "need to remember they didn't win the general election either" and that each side had an "obligation" to stick to the deal.

Some hostile Tories have accused the junior coalition partner of resorting to "blackmail" after a senior aide to Mr Clegg said the Lib Dems would block parliamentary boundary changes in retaliation.

But Business Secretary Vince Cable insisted his party was not issuing any threats or thinking of quitting the coalition and played down the prospects of defeat as government whips battled to control the revolt.

Senior Tory David Davis said most of his colleagues saw Lords reform however as "a sop to the liberals to give them something to be elected to when they get wiped out at the next election".

"If the vote had been taken on Friday they would have lost hands down. If they choose to prioritise this is and try and appease the Liberals, that's what happens," he said predicting the policy would be scuppered.

Mark Harper, the Tory minister in charge of steering the legislation through the Commons, said the timetable was "very sensible" and expressed confidence that defeat would be avoided.

"These are very Conservative proposals, in our manifesto, something Conservative colleagues ought to be able to support," Mr Harper told Murnaghan on Sky News - pointing out a mostly elected Lords had been policy since 1999.

At present the government is offering 10 full days of debate on the floor of the House.

Mr Harper has accused some rebels of playing "silly games" by pointing to the fact that the coalition agreement only contained a commitment to develop plans to reform the second chamber.

The team that drew up the deal was "very clear that what they were committing both coalition parties to do was to deliver on House of Lords reform, to deliver on nothing more than was in both of our manifestos".

The legislation, central to the Lib Dems' agenda in the coalition, would introduce an 80% elected Upper House and slim down membership from 800 to 450.

It would finally complete the removal of hereditary peers and replace 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.