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A referendum on changing Westminster's voting system will take place on 5 May after MPs finally managed to get their bill through Parliament.

Legislation authorising a referendum received royal assent late on Wednesday night after a stand-off over the issue.

Peers had proposed only making the referendum binding if 40% of the public took part, but the government managed to defeat that measure by 68 votes.

It had to be approved by Thursday for the referendum to happen in May.

The House of Lords eventually backed down shortly after 2300 GMT when a Labour amendment urging MPs to think again on the issue was defeated by 221 to 153 votes.

Lords leader Lord Strathclyde had earlier urged a packed House to back down and "respect the will of the elected Chamber" over the issue.

The elections watchdog had said the bill must get Royal Assent by 26 February to enable enough time for the referendum to be staged on 5 May.

But the situation was complicated by the fact the Commons will rise for a 10-day recess on Thursday.

The issue of the 40% clause was passed back and forward between the two Chambers in a effort to find a resolution to the dispute in a parliamentary process known as "ping-pong".

Earlier on Wednesday, peers voted to reinstate the 40% threshold clause by 277 to 215 - with Lord Tebbit, Lord Lamont, Lord Lawson, Lord Howe and Lord Mawhinney among Tory peers to rebel against the government.

This prompted accusations of "betrayal" by the former Lib Dem leader Lord Ashdown.

"We have delivered, in full, our side of the deal - the Conservatives seem unable to deliver theirs," he told the ´óÏó´«Ã½.

Ministers argued voters should be the ones to decide the outcome of the poll but critics said it was right for Parliament to determine what should happen if turnout is below a certain level.

Labour peer Lord Rooker, who proposed the original amendment, said it set a dangerous precedent for future referendums.

Peers earlier agreed not to defy the government on another sticking point relating to the size of constituencies.

An amendment by cross bencher Lord Pannick calling for constituencies to be allowed to deviate from the standard size by 7.5% - as opposed to the 5% margin sought by ministers - was defeated by one vote.

The Conservatives agreed to a referendum over whether to replace the first-past-the-post system for electing MPs with the alternative vote (AV) as part of their coalition negotiations.

The AV system allows electors to rank candidates in order of preference.