Ms Merkel will leave her post as German Chancellor later this month after 16 years in office. The 67-year-old has remained distanced on the campaign trail ahead of the vote on September 26 and is yet to fully endorse her party’s candidate.

       Armin Laschet, leader of the Christian Democratic Union (CDU), is the nomination for the CDU and its Bavarian sister party, the Christian Social Union (CSU).

       Ms Merkel has so far refused to say who she is voting for and pollsters claim this could swing the vote towards the opposition.

       Hermann Binkert, the head of polling firm INSA, said: “A considerable part - around a third - of the CDU voters voted for the party because of Merkel.

       “She has mobilised many green voters to vote for the Union.

       “If Merkel were to promote her party and Armin Laschet out of conviction, that would help both of them a lot.”

       The latest polls suggest Germany's Social Democrats (SPD), led by Olaf Scholz, has opened up a five-point lead.

       German government spokesman Steffen Seibert was asked again this week who Ms Merkel would be voting for – but refused to be drawn.

       He told German newspaper BILD: “The elections in Germany are free, equal and secret.

       The latest Infratest dimap poll suggested support for the SPD is at 25 percent, with the conservative CDU/CSU bloc on 20 percent.

       The Greens were on 16 percent, the liberal Free Democrats on 13 percent, the far-right Alternative for Germany on 12 percent and the far-left Linke on six percent.

       The last rally for the CDU/CSU party is scheduled to take place on September 24.

       Germany will head to the polls 48 hours later.

       (Additional reporting by Monika Pallenberg)