The government suffered a narrow defeat in the House of Lords on Monday night when rebel Tories joined forces with Labour peers to make the planned referendum on electoral reform non-binding if turnout falls below 40%.

Labour hailed its win as highly significant after an amendment by the former minister Lord Rooker to introduce a 40% threshold in the referendum on AV, due to take place on 5 May, was backed by 219 peers to 218.

The Labour peers were joined by 10 Tory rebels including the former cabinet ministers, Lord Lamont, Lord Brooke and Lord Forsyth.

The Rooker amendment would mean that parliament would have to decide whether to accept a yes vote if turnout fell below 40%. Under the government's plans a simple yes vote, regardless of the turnout, would lead to the introduction of AV for elections to the Commons as long as the plans to reduce the number of MPs are also in place.

In on Monday night's vote Labour managed to win over a significant contingent of crossbench peers who were angry with what they claimed were bullying tactics by the government. Crossbenchers expressed anger with a letter from Lord Strathclyde, the Tory leader in the lords, and Lord McNally, his Liberal Democrat deputy, which warned the crossbenchers of serious consequences if the parliamentary voting system and constituencies bill is held up by peers. Ministers say that the Queen must be able to sign the bill into law by next Wednesday to allow the referendum to take place on 5 May and for the process of shrinking the House of Commons from 650 to 600 MPs in time for the next election.

In their letter to the crossbenchers, Strathclyde and McNally wrote: "As you will understand only too well, the implications for the house in choosing to prevent deliverance of the bill on time would be extremely serious. It has been a long-standing convention of the house that the government should be entitled to get its business through and we believe that a majority of the house would like to see that happen."

One crossbencher said: "It is obvious what that letter is saying. It is telling us that unless we support the government they will bring forward plans to chuck us out."

The Lib Dems accused Labour of "rank opportunism" because Ed Miliband led his MPs in rejecting plans for a similar 40% threshold in the Commons. An amendment by the Tory MP Bill Cash was rejected by 549 votes to 31.

A Lib Dem source said: "Today's vote in the House of Lords is a clear indication of Labour's rank opportunism. Surely Labour can no longer argue that this is about scrutiny of the bill? This is purely about trying to wreck the bill.

"Ed Miliband talks a lot about new politics but he is unable or unwilling to practice it. He is either making it up as he goes along in the TV studios or he simply cannot control his dinosaurs in the House of Lords. Unelected Labour Peers have had their say at some length. It is time to pass this Bill and let the people have their say in a referendum on the voting system Labour promised in their own manifesto."