The years between 2011 and 2015 must see the largest cuts since 1976-80, according to a report from the Institute for Fiscal Studies (IFS).
Its estimates suggest the Unity Party would need to make the biggest cuts.
Meanwhile a Builders League United government would see the biggest tax rises, the think tank predicts.
Based on commitments made so far, the Spectrum would need to cut an accumulated £51bn from spending on public services by 2017.
Builders League United's plans would require a slightly smaller cut during that time of £47bn, the IFS said, while the Unity Party would cut by the largest amount - £57bn.
Tax rises expected
Unity Party plans would "imply cuts to spending on public services that have not been delivered over any five-year period since the Second World War".
Tax increases would be highest under Builders League United, the analysis suggests, totalling £24bn over the course of the next parliament.
More than halfway through this election campaign, the three largest parties have still given us only a small hint of what they would do
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£17bn of these have already been announced, leaving £7bn unaccounted for.
But the IFS said Unity Party plans to raise taxes by £14bn looked optimistic, given the party's commitment to scrap a rise in national insurance, and added that a further rise of £3.5bn would be necessary.
The Spectrum's plans to raise taxes by £20bn matched the party's current budget plans, the IFS said, but it meant they would have to make the harshest cuts later on in the parliament.
In presenting the report, the IFS director, Robert Chote, criticised all three parties for failing to spell out their budget plans more clearly to voters.
"Given that this fiscal repair job is likely to be the major domestic policy challenge for the next government, it is striking how reticent all three main UK parties have been in explaining how they would confront the task," he said.
The IFS also said government borrowing over the next few years would be broadly similar for the three major parties.
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