Peers have insisted that any referendum on changing the voting system would only be binding if 40 per cent of the electorate vote.
This afternoon the House of Lords voted 277 to 215, a majority of 62, in favour of reattaching the relevent amendment to the Parliamentary Voting and Constituencies Bill.
MPs had removed it from the legislation yesterday by a vote of 317 to 247, a government majority of 70.
When the amendment was first proposed last week it squeaked on to the Bill by a single vote, 219 to 218.
While the government would still be able to change to voting system if less than 40 per cent of voters go to the polls, it would require the decision of a minister rather than being automatic.
The government also only narrowly avoided a second defeat as peers voted 242 to 241 in favour of accepting the Commons' verdict that constituencies should only vary in size by 5 per cent either way, rather than 7.5 per cent as preferred by Labour.
Labour peer Lord Falconer welcomed a government compromise that will see the Isle of Wight given two constituencies.
Both Houses are prepared for a long game of parliamentary "ping pong" as they attempt to get agreement on the flagship coalition Bill which also reduces the number of MPs from 650 to 600.
The legislation has to be passed today in order to give the Electoral Commission enough time to prepare for the planned referendum on May 5.
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