The Liberal Democrats have made their boldest step yet to distance themselves from Conservative economic policy when David Laws, a former deputy to George Osborne, described the chancellor’s public spending plans to 2020 as a political suicide note. They were so severe, he said, that they made Thatcherism look like a policy devised by Tony Benn.

The language deployed by Laws is startling since he is seen as being on the right of the Liberal Democrats and for a short period was Treasury chief secretary when the 2010 spending review was prepared. At one point when not in government he called for public spending to be cut back to 35% of national income.

But in an intervention on Sunday, he said the Conservatives had made a politically catastrophic error in trying to draw up rightwing economic dividing lines that will require cuts in some Whitehall departments of a quarter. Conservative voters who value the armed forces, police and prisons will be worried by the “mind-boggling” consequences of the cuts, Laws said.

He predicted “panic” in the Tory ranks and more jockeying for position among senior ministers who want to replace David Cameron if opinion polls do not show his support picking up.

Laws accused Osborne of a “huge policy and strategic blunder” on a par with Michael Foot’s 1983 manifesto – a byword for electoral disaster that was dubbed “the longest suicide note in history”.

Laws predicted that the Tory plans would “unravel” under the scrutiny of an election campaign or would prove to be a “rightwing version of Michael Foot’s manifesto”.

Laws will face charges of hypocrisy since he was one of the architects of the original austerity package in 2010, but with the Liberal Democrats now primarily in a battle to hold on to seats where the party’s chief challengers are Tories, its strategic goal must be to depict the Tories as extreme. That way they can hope to win over centrist Conservatives and secure Labour supporters’s tactical.

Laws said: “This will be seen to be a very extreme and very rightwing suicide note because all those people who care about the education service, about the police, about the armed forces … will see that the plans they have put forward are hugely damaging and dangerous.”

He added: “In order to deliver this scale of savings you are talking about having to find cuts in the unprotected budget of around a quarter in the next parliament.”

Even then the Conservatives will only achieve their goals if they find welfare cuts on a scale that “would hugely increase the levels of poverty in the country”.

The Liberal Democrats have – like the Tories – signed up to clearing the deficit in day-to-day spending in 2017-18, but say this must be achieved by some tax rises, and not, as the Tories plan, only though spending cuts. Nick Clegg’s party does not support the £12bn of welfare cuts proposed by Osborne in the first two years of the parliament.

The Liberal Democrats, like Labour, would allow extra borrowing for capital investment, and unlike the Tories would not seek to achieve a surplus on the current and capital account surplus by the end of the next parliament.

Chris Leslie, shadow chief secretary, dismissed Laws’ intervention, saying: “Nobody will fall for this desperate attempt by the Lib Dems to distance themselves from the government they are part of.

“Two years ago David Laws called for deep cuts to public services to reduce the size of the state to 35 per cent.

“And Nick Clegg and Danny Alexander signed off spending plans in the autumn statement that would do just that and take public spending back to a share of GDP last seen in the 1930s.”