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Nick Clegg has cancelled a trip to South America as the government battles to secure legislation needed to hold a referendum on the UK voting system.

The deputy prime minister had been due to travel to Brazil on Sunday on a four-day visit to promote trade.

But he has pulled out amid doubts that the bill to enact the Alternative Vote referendum will become law by 24 February - as needed for a 5 May poll.

The bill has been held up in the House of Lords amid prolonged debates.

The House of Lords is due to take its half term break from next Wednesday - and the Commons from the next day - so, unless they decide to sit through the planned recess the deadline for the bill to complete its parliamentary stages is 16 February.

Downing Street has insisted there are "no jitters" about the bill.

The government has accused Labour peers of deliberately trying to obstruct the passage of the Parliamentary Voting System and Constituencies Bill - which also includes measures to redraw constituency boundaries and cut the number of MPs by 50. Ministers also accuse Labour peers of undermining the role of the Lords.

The opposition has said the bill did not get sufficient scrutiny in the Commons and says it is doing its duty to give it proper analysis.

Ministers suffered a fourth defeat on the substance of the bill on Wednesday as peers from all sides attempted to find a way forward and end weeks of acrimony over the issue.

Mr Clegg is in charge of the coalition's constitutional change programme and responsible for getting the Bill through Parliament.

Peers have been debating the proposed bill for nearly three weeks and will begin its third reading on Monday.

The proposed legislation must then return to the Commons for final approval by MPs before it can receive Royal Assent and get on the statute book.

The elections watchdog has said this must happen by 24 February for the poll to be held, as intended by the government, on 5 May.

However, the Lords breaks up for a recess on 16 February - and the Commons on the 17th - so the deadline is seen as being the 16 February.

Mr Clegg had been due to meet the presidents of Brazil and Mexico on his trip but he will now remain at home to try and steer the bill through its final stages.

A government spokesman said "it is an important bill and Nick Clegg and the other ministers on the trip are needed in the Commons next week".

The government was committed to the bill and intended to proceed with plans to hold the referendum on 5 May, the spokesman added.

Peers defeated the government over plans to equalise constituency sizes on Wednesday, calling for the number of voters permitted in any constituency to be within a 7.5% margin of the proposed 76,000 standard "in exceptional circumstances" - not 5% as proposed by ministers.

This followed a defeat on Monday over rebel calls for the referendum not to be binding unless 40% of the electorate take part.

MPs have the chance to override both of these measures when the bill returns to the Commons next week.

The Conservatives conceded a vote on whether to retain the existing first-past-the-post system for electing MPs or replace it with the Alternative Vote during coalition negotiations with the Lib Dems.

Mr Clegg has pushed for the poll to be held on 5 May, the same day as devolved elections in Scotland, Wales and Northern Ireland and council polls in England.

The head of the Electoral Commission, Jenny Watson, said they were "ready to go" if the bill got Royal Assent, although she said they would need more clarification if the 40% threshold was to stand.

She told a committee of MPs there would have to be agreement on things such as how the total electorate was measured, and on whether spoilt ballot papers would be counted.