The French Socialist party says it will fight to introduce a flagship wealth tax to raise revenue by targeting France’s richest people, as the divided parliament prepares to begin debating next year’s budget.

Boris Vallaud, the head of the Socialist party grouping in parliament, said on Wednesday that taxing very high-wealth individuals in France was “one of our principal battles and we’re going to put all our energy into it”.

The centre-left party is seeking to capitalise on the unprecedented political crisis that has weakened the new centrist prime minister, Sébastien Lecornu, after he resigned last week and was reappointed four days later.

The Socialists are now proposing the “Zucman tax”, named after an idea put forward by the French economist Gabriel Zucman, under which a 2% levy would be imposed on wealth above €100m, which would affect about 0.01% of taxpayers.

The prime minister faces his first major test on Thursday morning when two votes of no-confidence are expected to be held in an attempt to topple his minority government, one put forward by the leftwing La France Insoumise and the other by Marine Le Pen’s far-right National Rally.

Lecornu is likely to survive after the Socialist party decided to stand back and not join the vote to bring him down.

The Socialist party leadership said it would instead pin its hopes on Lecornu’s promised parliamentary debate of the contested 2026 budget. The budget, which aims to reduce the deficit to 4.7% of GDP from this year’s 5.4%, hinges on a squeeze of more than €30bn, including cuts to corporate tax breaks, tighter rules on social welfare contributions, and new taxes.

To secure the Socialists’ decision not to topple him, Lecornu made a significant concession to suspend Macron’s landmark pension changes, which were forced through in 2023 amid large street demonstrations and which had begun to raise the pension age from 62 to 64 over several years.

The Socialists, aware that they would hold deciding votes in any attempt to topple the government, now want to use that leverage to push some form of wealth tax, despite strong opposition from Macron’s centrists.

Their leader, Olivier Faure, wrote on social media: “In the forthcoming debate, we on the left will be working together to defend the Zucman tax and public services and to protect the poorest.” He said if the Zucman tax was not passed “we’ll have other proposals on a whole range of ways of targeting large fortunes, high wealth and big companies”.

In an interview with Reuters earlier this month, Zucman said the wealthiest households in many countries paid less income tax than most citizens, and in France that gap was especially stark. “Firstly, billionaires pay virtually no income tax in France, and secondly their wealth has grown particularly rapidly over the last 15 years,” he said.

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Zucman estimated the proposed 2% tax would affect only 1,800 households but raise up to €20bn annually, helping to reduce France’s budget deficit, estimated at 5.4% of GDP – the euro zone’s biggest.

A group of seven leading economists, writing in the French newspaper Le Monde this month, said the tax would be likely to yield closer to €5bn and could cause wealthy people to leave France.

Lecornu has publicly opposed the tax, but in his first policy speech to parliament on Tuesday he said he would ask for an exceptional levy on large fortunes. The prime minister said suspending the pension changes would have a cost to the public purse, and the shortfall would have to be made up.

Reuters contributed to this report