Ed Miliband today opens the door to future co-operation with the Liberal Democrats as he calls on disaffected ministers in Nick Clegg's party to quit the cabinet and join Labour in a fight against rightwing Tory policies.

The Labour leader, whose party now faces an uphill struggle to secure a Commons majority following the collapse of its vote in Scotland, says he will "work with any Liberal Democrats" against the Conservatives and their plans on the NHS, education and the economy.

Following the Lib Dems' disastrous showing in Thursday's council elections and the AV referendum – and amid increasing Lib Dem anger with David Cameron over campaign tactics – Miliband says it is "late, but not too late" for Clegg's ministers to jump ship.

"Do they want Tory policies or progressive ones?" he asked. "If they are in favour of new politics they should start by keeping their promises and reflecting the will of those who put them into parliament. If they are not in favour of these Tory policies they should stand up for what they believe or leave the cabinet. They can come and work with us. My door is always open."

While Miliband insists that his objective is still a majority Labour government and his immediate focus is on working with the Lib Dems against Tory policies, his overtures suggest that the party is prepared to plan for the possibility of a Lab-Lib deal after the next election.

Sympathising with the Lib Dems over how the Tory-backed no campaign behaved during the referendum campaign Miliband said: "The campaign on AV was a showcase for old politics at its very worst. Lib Dems have to work out which side they are on. Do they want to be on the Conservative side, backing the Conservative-led government, or on the progressive side? It really is time for them to make up their minds."

Sources close to Clegg stressed that the Lib Dems' central objective was now to stop the Tories winning an outright majority at the next election – and for them to have an option to team up with Labour.

As the Lib Dems tried to come to terms with losing nine councils and 695 council seats, as well as burying hope of electoral reform for a generation, there were bitter recriminations over the no campaign's targeting of Clegg.

Lib Dem business secretary Vince Cable, who is known to feel closer to Labour than the Tories, said he would continue to support the coalition, but added: "Some of us never had many illusions about the Conservatives, but they have emerged as ruthless, calculating and thoroughly tribal."

Former Lib Dem leader Lord Ashdown went further: "We are bloody but unbowed. We have been here before and have always confounded the prophets of doom. But what makes this particularly hard to bear is the widespread, and in my view justified, feeling in the party that the Tories were either allowed to – or encouraged to – join a national vilification of our party leader and seem to have benefited from that."

Lib Dem peer Lord Oakeshott said it was time chief secretary to the treasury Danny Alexander rethought the political consequences of his role as number two to chancellor George Osborne. "He doesn't need to be a royal bodyguard, throwing himself in front of every bullet heading for Osborne."

On Monday Labour will seek to expose Lib Dem discomfort within the coalition by calling on its MPs to support an opposition motion opposing the government's NHS reforms, which are strongly opposed by Lib Dem activists. Then on Wednesday it will ask the Lib Dems to support a series of Labour amendments to Michael Gove's education bill, including one insisting that all teachers in schools be fully qualified.

Neal Lawson, chair of the centre-left thinktank Compass, said it was right for Miliband to be thinking of working with the Lib Dems. "The worry is that if Labour is flatlining when the Tories are cutting services, its support will collapse when they cut taxes before the next election. Ed Miliband knows he can't win a two versus one election against the Tories and the Liberal Democrats. And the best he can hope for right now is a progressive coalition government with a Liberal Democrat party that has dumped Nick Clegg. He needs to prepare the ground for such a campaign and coalition now."

Writing in today's Observer, Lib Dem president Tim Farron, seen by some as a potential successor to Clegg, calls on his party to fight its corner more assertively while keeping faith in the coalition. Yesterday the leader of the Scottish Liberal Democrats, Tavish Scott, resigned. He said: "Thursday's Scottish general election result was disastrous and I must and do take responsibility."Last night a senior source in the campaign for the alternative vote admitted they knew "very early on" that there was no chance of winning the referendum and that Clegg had become part of the problem: "Every time Clegg spoke about AV our polling numbers went into free-fall. We knew from very early on, before the new year, that we couldn't win, our message wasn't getting through and the Liberal Democrats in the whole were worse than useless. Clegg was toxic and everything [Chris] Huhne did in criticising the Tories just put the attention on the political spat – made it a Clegg versus Cameron affair. Utterly unwinnable.

"We even brought in an advertising man to save us. He came up with the idea of constructing a giant pin-striped bottom to take around the country for people to throw things at as a way of illustrating that AV makes MPs work harder. It was desperate stuff.

• The following correction was printed in the Observer's For the Record column, Sunday 15 May 2011. This article said that on 5 May the Liberal Democrats had lost 695 council seats and nine councils. They actually lost 842 council seats and 11 councils.