Tensions inside the coalition government are at new heights after a Liberal Democrat cabinet minister called on voters to form an anti-Tory alliance in Thursday's referendum on electoral reform in order to deprive the Conservatives of power.

In an extraordinary intervention, Chris Huhne, the energy secretary, has shattered any remaining semblance of cabinet unity by insisting that the referendum is an opportunity for the country's "progressive majority" to back change and avoid a repeat of the "worst excesses of the Thatcher government".

In a joint article in the Observer, signed by Labour's shadow business secretary, John Denham, and the leader of the Green party, Caroline Lucas, Huhne and the others argue that the Conservatives were able to monopolise power for much of the 20th century because of an "unfair" first-past-the-post system.

"Britain consistently votes as a centre-left country, and yet the Conservatives have dominated our politics for two-thirds of the time since 1900," the three say. "On only two occasions in that long century – 1900 and 1931 – have the Tories won a majority of the votes. No wonder David Cameron says the current system has 'served us well'."

They add: "For those who weren't well served by the Tory 20th century, fair votes matter. They matter for the millions of voters who suffered the worst excesses of the Thatcher government despite more than 54% repeatedly voting against her."

The remarks from Huhne amount to a declaration of war by one of Nick Clegg's most senior ministers on the Tories' record in government under Margaret Thatcher – but also an attack on a political philosophy the trio suggest still drives Cameron's party.

They will provoke fury in Conservative ranks, particularly among rightwing Tories who already resent the Lib Dems' presence in the coalition and their influence over policy.

On Thursday, after a campaign that has turned increasingly bitter as the Lib Dem hopes of a yes vote have faded, people will be asked whether they want to ditch the current first-past-the-post system – under which the candidate with most votes wins – in favour of the alternative vote (AV).

Under AV, if no candidate receives more than 50% of the votes, the one with the least votes is eliminated and their second preferences are distributed among the rest. This process continues until one candidate has a clear majority.

Cameron opposes change while the Lib Dems strongly back it, arguing that under AV any MP would have to have the support of at least half of the voters to be elected.

The referendum was a central Lib Dem demand in coalition negotiations with the Tories following last year's general election. Recent polls suggest, however, that the no campaign is ahead, although pollsters admit the result is difficult to call because turnout is expected to be low and many people remain undecided.

In an interview with the Observer's chief political commentator, Andrew Rawnsley, Clegg accuses the no campaign, backed by Cameron, of spreading "ludicrous bilge" about AV with the deliberate intention of misleading the public.

Blaming the prime minister and the chancellor, George Osborne, Clegg says that both men "became very worried that the right of the Conservative party would react very badly if AV wasn't defeated and they basically decided to throw the kitchen sink at the referendum".

As comedian Eddie Izzard begins a three-day national tour to promote AV, the Labour leader, Ed Miliband, will also renew his call to voters to back AV. "I urge people who are making up their minds to vote for a system which will make for a more accountable House of Commons, fairer votes and a change in our political culture."

Miliband will argue that the Tories, having failed to gain a Commons majority last May, lack an "electoral mandate" for savage spending cuts and radical plans for the NHS, and will urge voters to reject them in Thursday's council elections and polls for the devolved assemblies. "This week people are being given a chance to deliver a verdict on a year of a Conservative-led government and the willing participation of the Liberal Democrats within it," the Labour leader will say in a speech on Monday.

"Labour has changed as a party since the last election. There is further to go, but we are a party people are coming towards, not turning away from. Thursday is a chance for people to vote for what they value by electing Labour councillors, MSPs and Welsh assembly members."

The Yes to AV campaign will reveal figures on Sunday showing that MPs who enjoy jobs for life in "safe seats" delivered under first-past-the-post earn almost twice as much in outside earnings as those in more marginal seats.

Huhne, Denham and Lucas also lump the Tories together with the British National party as opponents of AV.