Today the coalition publishes its bill on Lords reform, which is hugely controversial among Conservative MPs. Whether or not the "programme motion" is agreed (Labour having indicated that it will vote against) the bill will spend many days in committee on the floor of the Commons. It could end that process in a very different shape to that in which it began, whatever the party leaders say.

Blair's defeats in the Commons are a precedent that Cameron and Clegg should remember. In the end, it is what the Commons wants that matters, and party leaders do not always get their way. Blair was defeated by backbenchers over his plans for 90 days detention of terrorist suspects, despite his large Commons majority. Likewise the Commons legislated for a ban on foxhunting despite him having voted against it himself. Tempers are running high in the Commons and the outcome is very uncertain indeed.

Lords reform expert Dr Meg Russell, Deputy Director of the Constitution Unit at UCL said: "There is now a bill which both Cameron and Clegg support, and Labour also supports reform in principle. But ultimately it is the Commons that will decide. MPs on all sides hold strong views and are clearly not afraid of being assertive. There are many likely changes once the bill gets into committee. Government defeats seem inevitable."

Key issues on which defeat is possible include:

• the referendum (supported by both Labour and the joint committee on the draft bill);
• whether the chamber is 80% or 100% elected (in both Labour and LibDem manifestos);
• the electoral system and boundaries;
• the length of elected members' terms and whether these are renewable;
• the presence of the bishops;
• transitional arrangements, and members' pay.

But the biggest issue of all is likely to be the chamber's powers, and these clauses have had no scrutiny as yet. The government has amended the bill to placate backbenchers, setting out that the Parliament Acts still apply, and therefore the Commons remains the senior chamber. But for many MPs this won't be enough, as the Lords’ power is currently limited by concerns about its legitimacy as an unelected chamber. If it is to be elected, this will change.

Meg Russell said: "Lords reform has been on the agenda for 15 years, and we have had numerous reports and endless deliberation on changing the chamber's composition. In contrast these issues of powers are only just starting to be debated. They alone could result in the bill getting completely bogged down".

Notes for editors:

• Meg Russell is available for interview today and tomorrow on 0207 679 4998, meg.russell@ucl.ac.uk. Her research on the House of Lords is at: http://www.ucl.ac.uk/constitution-unit/research/parliament/house-of-lords
• Our Press Officer, Brian Walker can contacted on 07802 176347, williambrianwalker@gmail.com.
• The Constitution Unit is an independent research centre based in the Department of Political Science at University College London.