Labour leader Ed Miliband told the Yes to the Alternative Vote campaign at a private meeting that he would not share a platform with Nick Clegg during the referendum campaign. He said Clegg's name had become so toxic with Labour voters he would put off potential AV backers.

Miliband said at last week's meeting he was willing to share a platform with other Liberal Democrats, including Charles Kennedy, Lady Williams and Lord Ashdown. One source said: "His position is pretty well ABC – Anyone But Clegg."

A Lib Dem spokesman stated that he was surprised by the suggestion, saying that in private and public Miliband had told the party he would share a platform with Clegg. "It feels like this is a failure of leadership on his part and he has been defeated by the most tribalist elements in his party," the spokesman said.

But the Yes campaign director, Lord Sharkey, accepted Miliband's arguments at the end of the meeting attended by aides on both sides last week.

Sharkey, a Lib Dem peer appointed by Clegg, masterminded the Lib Dems' 2010 general election campaign. He appears to see Miliband's decision as a pragmatic acceptance of polling reality.

The Yes campaign has been consistently arguing that its campaign will not focus on politicians, but it would have been a coup if the leaders of the two centre-left parties had shared a platform.

The No campaign is making Clegg its chief target, urging the Yes campaign to come clean about its Lib Dem links. The No campaign claims five out of six members of the Yes steering committee have worked for or supported the Liberal Democrat party in the past 12 months.

The Lords will tomorrow resume the battle over the bill introducing the referendum on AV, with Labour peers trying to join forces with some cross-benchers to demand changes to the section on redrawing constituency boundaries. Further all-night sittings are likely to see the bill passed by early next week, the final deadline for the electoral commission to prepare for a 5 May referendum.

Nick Clegg is due to publish a government blueprint for Lords reform in the next fortnight. But Lord Falconer, the former Lord Chancellor and Labour peer leading the scrutiny of the referendum bill, claimed Clegg is likely to find his efforts to reform the Lords frustrated.

He said: "I would be absolutely amazed if there is a centimetre of political will on the part of the Conservatives. I think the Liberal Democrats may be pinning their hopes on false friends."

Falconer added: "Suppose the AV referendum was lost and Lords reform was kicked into the long grass – what would be said to have been the Liberal Democrats' contribution to this government?"