The referendum on changing the way MPs are elected could be delayed beyond 5 May, the BBC has learned.

The vote on bringing in the alternative vote system could be postponed because Labour peers are holding up the progress of the bill which would enable it to happen.

Peers have spent eight days discussing the bill - with more scrutiny planned.

It would have to become a parliamentary act by mid-February for the referendum to take place on time.

MPs have backed plans for the public vote on 5 May, despite concerns that combining it with devolved and local elections would detract from these polls and potentially confuse the electorate.

Peers have spent eight days discussing the Parliamentary Voting System and Constituencies Bill in its detailed "committee stage", with two more days of meetings scheduled for next week.

Warning letter

Last Monday the Lords spent five-and-a-half hours debating - but not voting on - two Labour amendments, a debate conducted almost entirely by the party's peers, many of them former MPs.

WHAT IS ALTERNATIVE VOTE 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. Vote reform: Where parties stand Q&A: Alternative Vote referendum

However, Labour peers deny they are trying to delay the bill, insisting they are instead trying to improve it.

They say ministers could get the parts of the legislation relating to the referendum through quickly if they separated that from the more controversial elements which pave the way for constituency boundary changes and a reduction in the number of MPs.

But the government has made it clear they are not prepared to do this because both issues relate to how MPs are elected to Parliament. The deal was also at the heart of the coalition agreement with Liberal Democrats agreeing to boundary changes that could help the Conservatives in return for getting a vote on electoral reform.

The crunch will come next week. The Government has scheduled an extra three days to complete the committee stage of the bill in an attempt to prevent opposition peers delaying it further. Government peers are threatening to keep the Lords sitting through Monday night to get through all the amendments, with some already promising to come with their sleeping bags

If the bill is delayed further there will not be enough time for it to get through its remaining legislative hurdles and become law by 16 February, the deadline set by the Electoral Commission watchdog which will oversee the referendum.

The commission has written to ministers to inform them that there will not be enough time to prepare for the ballot unless the legislation receives royal assent by this date.

Under the AV system - which would replace first-past-the post elections - 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