Historic losses for Germany’s two governing parties and a Green surge in Sunday’s European elections have ratcheted up the pressure on Angela Merkel’s embattled coalition government.

Both of the country’s established centrist parties slumped to their worst result in the postwar era as voters from each side of the political divide switched support to a buoyant Green party, emboldening critics who would like to pull the plug on the current power-sharing arrangement in Berlin.

While the alliance of Merkel’s conservative Christian Democratic Union (CDU) and its Bavarian sister party, the CSU, still emerged top with 28.9% of the vote, it shed five seats.

Meanwhile the centre-left Social Democratic party (SPD), junior coalition partner in the national government, endured a humbling evening.

Not only did the SPD’s share drop to what party leader Andrea Nahles called an “extremely disappointing” 15.8%, the world’s oldest social democratic party was also pipped to second place by a Green party that gained a historic 20.5% of the vote.

At state elections in Bremen also taking place on Sunday, the SPD failed to emerge as the strongest party for the first time in more than 70 years, losing out on the top spot to a political newcomer who has only been a member of the CDU for a year.

With the German centre-left continuing to haemorrhage votes while sister parties in Denmark and Spain have managed to reverse their fortunes, pressure is mounting on Nahles to step aside as leader.

Former deputy chancellor Sigmar Gabriel appeared to call for a change at the top of his party when he told news agency DPA that “those in Berlin who have brought about the current state in personnel now have to take responsibility”.

Florian Post, a Bavarian Social Democrat MP, was more blunt when he commented that “we have more than 150 delegates in the Bundestag, and almost all of them could do a better job than Andrea Nahles”.

While critical voices inside the conservative CDU were more muted, Sunday’s results also posed hard questions for the party that has governed Germany for the last 14 years.

Not only did the CDU lose more than a million voters to the Green party, the party led by Annegret Kramp-Karrenbauer also faced troubling losses in the former states of socialist East Germany.

In Brandenburg and Saxony, where citizens will vote in state elections in the autumn, the CDU was beaten into second place by the rightwing populist party Alternative für Deutschland (AfD). Across Germany as a whole, the AfD only made minor gains in its second European election, with party leaders blaming the Austrian far right’s recent corruption scandal for their underwhelming result.

An internal CDU analysis of Sunday’s vote, leaked to newspaper Die Welt, noted that the public was broadly critical of the “grand coalition” government, and the conservatives had failed to respond to a growing concern about the climate crisis.

Paradoxically, the same factors that are piling pressure on the CDU and SPD’s party leaders are likely to help prop up their ailing coalition in the short term. The SPD will be reluctant to trigger fresh elections that could see its poor results in the European elections recreated at a national level.

On Monday, Nahles rejected rumours about her resignation as SPD leader and hit back at her predecessors. “The 15% we are struggling with now were brought about over the last 15 years,” she said.