Speaking at the beginning of a key parliamentary debate to discuss the plans, which need a two-thirds majority to pass, Merz said "any further delay" in lifting defence spending "would be irresponsible".

"We must now take action to significantly increase our defence capabilities, and we must do so quickly," he said.

Pointing to the United States' deteriorating ties with Ukraine and its European allies, Merz said "we could be standing before a decision with deep consequences for our country's history".

Merz and his centre-right CDU/CSU alliance are currently in talks with the centre-left Social Democrats (SPD) on a possible coalition government after last month's general election.

Last week the two parties jointly presented plans for big spending packages, including exempting defence spending from Germany's constitutionally enshrined debt brake when it exceeds one percent of GDP, as well as on boosting infrastructure.

A vote on the plans is scheduled for Tuesday and Merz wants them to be approved quickly before the new parliament is convened.

In the new parliament, the far right and far left will together be able to block the plans.

SPD parliamentary leader Lars Klingbeil also urged MPs to back the government-in-waiting's plans: "When history knocks at our door, then we must open it, we don't know if we will have a second chance".