Germany's Greens may refuse to back plans by likely next chancellor Friedrich Merz for a massive increase in state borrowing, with a party source on Monday warning "approval is becoming less likely with each passing day".

The party said they would not support the conservative chancellor-in-waiting’s paradigm-changing plan to invest massively in defense and infrastructure.

Germany's Greens vowed to block plans by likely next chancellor Friedrich Merz for a massive increase in state borrowing but left room for compromise on Monday on measures to revamp the military and revive growth in Europe's biggest economy.

A massive plan to loosen Germany's debt rules and spend hundreds of billions on defence and infrastructure has run into potentially decisive opposition from the Greens, whose votes in parliament would be essential for the legislation to be approved.

Germany’s Green party rejected a debt-financed package that would unleash hundreds billions of euros in defense and infrastructure spending, potentially imperiling the flagship policy of chancellor-in-waiting Friedrich Merz.

Germany's Greens will negotiate hard before potentially giving their much-needed backing to debt reforms proposed by the conservatives and Social Democrats, parliamentary co-leader Katharina Droege said on Wednesday.

A massive plan to loosen Germany's debt rules and spend hundreds of billions on defence and infrastructure has run into potentially decisive opposition from the Greens, whose votes in parliament would be essential for passage.

The CDU and SPD, likely coalition partners in Germany's next government, want to relax the debt brake for spending on defense and infrastructure. But the Greens said this would not invest enough in climate protection.

Germany's Green party on Monday threatened to torpedo plans put forward by likely next chancellor Friedrich Merz to invest hundreds of billions in defence and infrastructure. The proposed constitutional changes to ease strict borrowing limits in the area of defence and establish a 500-billion-euro ($542-billion) fund for infrastructure were described as historic when they were presented last week.

Despite hefty criticism from the Greens, Germany's conservative CDU/CSU and centre-left Social Democrats (SPD) are still counting on striking a deal to pass a massive defence and infrastructure spending package.