“There are things that are strange to me, for example the visit of his daughter to Germany which was treated almost like a world event,” Mr Gabriel said on Saturday.

“This mix of politics with family and business smacks of nepotism and would be unimaginable here.”

Ms Trump’s appearance at an international women’s summit at the personal invitation of Mrs Merkel this week was hailed as a soft diplomacy coup for the German chancellor.

But Mr Gabriel made clear his disdain in an unusually outspoken interview with several German regional papers.

“It always bothers me when members of a family, who have never been elected, show up suddenly as official state representatives and are treated almost as if they were members of a royal family,” he said.

Ms Trump was jeered by the audience when she tried to defend her father at the women’s summit in Berlin on Tuesday.