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The House of Lords has ended a marathon sitting after talking for nearly 21 hours about proposed changes to the voting system.

It is the first of an expected series of all-night sittings, as the government attempts to force through plans for a referendum on voting.

Peers will resume later on Tuesday for more debate.

Ministers have until mid-February to ensure a referendum on the alternative vote system can go ahead on 5 May.

Labour objects to the part of the bill which would cut the number of MPs.

Downing Street said the Cabinet was "absolutely clear and united" that this bit of the bill would not be split off.

The PM's official spokesman said the government was clear that the original package, which was part of the coalition document binding the Lib Dems and Tories together in government, would remain.

He said: "Our intention is to have the referendum on 5 May".

The reason was that voters would only have to go to the polling station once - other elections are being held that day - and it was much cheaper, he added.

Board games and talks from celebrity Lords were arranged to entertain peers through the first of an expected series of all-night sittings on Monday.

The Lords Deputy Speakers have organised a rota until 1300 GMT on Tuesday, which would make the sitting, which began at 1538 GMT on Monday, one of the longest in living memory. Lengthy Lords sittings are also expected over the next two days.

The longest Lords sitting was on 10-11 March 2005, when peers sat from 1100 until 1931 the following day, debating amendments to the Prevention of Terrorism Bill.

Peers are considering in detail proposed amendments to the Parliamentary Voting System and Constituencies Bill.

AV, which would replace the first-past-the-post system for Westminster elections, allows voters to rank candidates in order of preference.

But Labour wants the bill to be split up, so that the planned referendum on changing to the voting system does not go through Parliament at the same time as proposals to alter the boundaries - and reduce the number - of MPs' constituencies.

The party argues that the coalition is trying to hinder its chances at future elections, but ministers say the changes to seats are needed to make the system fairer and cheaper.

Opening the peers' debate on Monday, Lords leader Lord Strathclyde said the bill had already spent too long going through Parliament, having first been introduced to the Lords last November.

He said: "The opposition have dragged their feet. They've had their fun."

He added: "The situation has become urgent because the Labour Party has decided to go on a marathon go-slow since we started the committee [to consider the bill]."

But Labour's Lord Falconer said: "This bill is motivated by party politics... It has been introduced without public consultation or pre-legislative scrutiny."

He added: "The bill runs to over 300 pages... It's unlikely in the extreme that, uniquely among bills, it cannot be improved further by this house."

The parliamentary authorities made arrangements for peers to stay over in the building overnight, including setting up camp beds for those needing to rest during the debate.

Linking the two issues of an AV and constituency changes was an element of the coalition negotiations, with the Tories determined to get boundary reform while the referendum was a central Liberal Democrat demand.

Lord Falconer said: "We urge the government to think again about splitting the bill... My experience is that widespread consultation will provide a solution."

Labour proposed an amendment to the bill to overrule the government's plans to reduce the number of MPs from 650 to 600.

But this was rejected without a vote, after a debate lasting four hours.

Under the AV system, voters rank candidates in their constituency in order of preference.

Anyone getting more than 50% of first-preference votes is elected. If no-one gets 50% of votes, the candidate with the fewest votes is eliminated and their backers' second choices allocated to those remaining. This process continues until one candidate has at least 50% of all votes cast.

The Parliamentary Voting System and Constituencies Bill, which has already been voted through by MPs, would need to gain Royal Assent by 16 February to allow the referendum - scheduled for the same day as Scottish, Welsh, Northern Ireland and local elections - to go ahead on time.