Media playback is unsupported on your device Media caption The prime minister says it is 'time to make progress' on reforms of the House of Lords

Prime Minister David Cameron has called for MPs to support Lords reform, as the government unveils its plans for a mostly elected House of Lords.

He said it was "time to make progress" after 100 years of attempts to transform Parliament's second chamber.

The cabinet agreed on Tuesday to push for 80% of the house to be chosen by voters. The number of peers is also expected to be almost halved from 800.

Mr Cameron is likely to face a rebellion by Conservative MPs.

Many Tory MPs believe constitutional change should not be a priority and up to 100 are expected to oppose the bill.

Labour is backing change, despite opposition from some of its MPs.

The proposals in the House of Lords Reform Bill include:

Cutting the number of members from 826 to 450

Peers serving non-renewable 15-year terms

Elections to take place every five years, with one third of seats up for re-election

Members to represent different regions

First elections to take place in 2015, then 2020 and 2025, with existing members being "phased out"

The remaining 90 members (20%) would be chosen by an Appointments Commission, on a non-party basis

The number of Church of England bishops in the Lords to be cut from 26 to 12

It would still be called the House of Lords, but members would not have the title "Lord", with parliament to decide on a new name for them

Changes to the Lords were promised by all three main parties at the last general election, but Deputy Prime Minister Nick Clegg and his fellow Liberal Democrats are seen as the main driving force behind the coalition government's plans.

At Prime Minister's Questions, Mr Cameron said a majority of MPs and the wider public were in favour of a mainly elected House of Lords and urged MPs to back the government's bill.

He said: "If those who support Lords reform don't get out there and back it, it won't happen. That is the crucial point."

'Constitutional monstrosity'

Mr Clegg said: "The coalition stands on the brink of an historic achievement. After more than a hundred years of debates, cross-party talks, green papers, white papers, command papers and a Royal Commission, we are finally introducing a bill to create a democratic and legitimate House of Lords."

The government had initially intended to cut the number of peers to 300, but this was raised to 450 after a committee of MPs suggested this might make Parliament's second chamber too small to scrutinise legislation properly.

Analysis All three party leaders support Lords reform. All three parties are being whipped to back it. All three parties included it in their election manifestoes. So in theory it should be a political cake walk. But, of course, it won't. History has shown that Lords reform is one of the most divisive, intractable and perilous of political undertakings. This time will be no different. For Nick Clegg it is a make-or-break moment. Succeed, and he can lay claim to an historic Liberal achievement. Defeat, and dark mutterings will start over his leadership. For David Cameron the risk is he prompts a major backbench revolt, souring relations with a large swathe of his MPs and possibly seeing his legislative agenda de-railed by endless days of debate on Lords reform. For Ed Miliband the danger is he risks angering many of his party's old guard opponents of Lords reform and being accused of playing political games by his qualified support for the Government's approach. But the bigger danger for all three leaders? The public will simply view Lords reform as a huge distraction from the many more pressing problems ordinary families are facing. Nick Robinson: Why reform may fail

But some opponents of the bill, including many on the Conservative benches, argue that elected peers could undermine the supremacy of the Commons by creating a rival chamber.

The bill is expected to receive a second reading in the Commons before MPs rise for the summer recess on 17 July. Ministers aim to complete its passage into law by next May. That timetable is far from certain, however, given opposition in both Houses.

Conservative MP Jesse Norman said he planned to rebel, adding there was "no question whatsoever" that he could support it.

He told BBC Radio 4's World at One: "This a constitutional monstrosity as a bill and it should never have reached the House of Commons."

Conor Burns, another Tory MP, said he would be prepared to risk losing his job as parliamentary private secretary to Northern Ireland Secretary Owen Paterson by opposing the bill.

After the second reading of the bill, MPs will be asked to vote on a "programme motion" limiting the amount of time the Commons can spend debating the changes.

Mr Burns told the BBC's News Channel: "If it is a disciplinary matter to vote in a way that was a free vote in previous parliaments... then so be it. I feel very strongly about this. This is about the constitution of the United Kingdom. We have never before in Parliament guillotined bills of such constitutional magnitude."

He said he expected a "significant" number of Conservative MPs to vote against a time-limited debate.

The prime minister's spokesman said Conservative MPs would be ordered to vote in favour of the government's proposals.

'More voters'

Labour leader Ed Miliband is also said to be facing a revolt from some of his senior MPs over his support for changes to the Lords, with former Home Secretary David Blunkett openly criticising the decision.

Fellow former Labour Home Secretary Lord Reid told the BBC's News Channel: "If anyone thinks that you will create a new class of 450 senators, with a term three times as long as MPs, with constituencies 10 times as big, with no constituency business to do... and that will not become the primary house, they are deluding themselves.

"It will not only rival the House of Commons; it will supersede it."

In the Lords, several peers questioned whether the bill would adequately protect the supremacy of the Commons.

Labour's Lord Richard, who chaired the joint committee on Lords reform, said the government should look at further ways of "buttressing" it as the primary chamber.

Former Conservative cabinet ministers Lord Gummer and Lord Forsyth both criticised the proposal for future members to be elected from a list of candidates, chosen by political parties, saying it would undermine the independence of members.

Labour's Baroness Symons questioned whether new members of the Lords "sent with the backing of millions of votes" from eight large regional constituencies and for 15 years would defer to MPs, elected with fewer votes for shorter five-year terms.

These new members, she said, would be "much more powerful than colleagues down the other end of the corridor".

But Lord Strathclyde, the leader of the House of Lords, defended the bill, saying the government believes a democratic mandate would give the Lords greater legitimacy and enhance its ability to revise legislation and hold the executive to account.