Angela Merkel’s deputy has attacked her handling of the migrant crisis, forcing the German chancellor to defend her refugee policy and firing the gun on the campaign for a general election next year.

Sigmar Gabriel, leader of the left-leaning Social Democrats and Ms Merkel’s junior coalition partner, called for a cap on the number of migrants entering Germany, saying the government had “underestimated the challenge” of integrating the 1m refugees who arrived in 2015.

His remarks show how battle lines are being drawn between Germany’s main parties ahead of next year’s election. They coincided with a poll showing that half Germans did not want Ms Merkel, who has been chancellor since 2005, to remain in the post after next year’s elections. She has refused to confirm whether she will stand again.