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Nikki Haley’s chances of becoming Donald Trump’s vice president are doubtful, his son has said, as he hinted he would lobby the former president “very aggressively” against her selection as his running mate.

Speaking at a Republican caucus in Des Moines, Iowa on Monday night, Don Jnr said his father was “probably… just trolling” when he told an audience last week that he had already selected his vice presidential nominee for the 2024 race.

Ms Haley has repeatedly refused to rule out accepting a job as Mr Trump’s running mate, pouring fuel on speculation they will run on a joint ticket.

Asked whether she would accept the job, she said on Sunday: “I don’t play for a second. I’ve never played for a second. I’m not going to start now.

“I’m not interested in being vice president. I’m running to be president and I’m running to win and we will.”

However, Mr Trump Jnr suggested she would have little chance of a spot on his ticket.

Asked by The Telegraph whether there was any prospect of a Trump-Haley campaign, he said: “I doubt that. I don’t know who it’s going to be. There’s a lot that has to come from that.”

On Tuesday, the former president told an audience of Republicans in Des Moines that he had already decided who his running mate would be, but would not reveal who he had picked.

“Well, I can’t tell you that really. I mean, I know who it’s going to be,” he said.

Mr Trump Jnr said there was “a solid chance [his father] was just trolling those people at the time” and “having some fun with it”.

“I think there’s probably a lot of credible options right now, and there’s a lot of people I would be fine with and a lot of people I would go very aggressively against,” he said.

Asked if Ms Haley was one of the candidates he would lobby against, he replied: “I probably wouldn’t be a huge fan. I’ve said that pretty publicly.”

His comments came as Mr Trump Jnr and Ms Haley appeared at the same caucus site, making the case for their respective campaigns to Republican voters.

Both repeated lines they had used on the campaign trail in Iowa, with her arguing that “you don’t replace Democrat chaos with Republican chaos” and him describing her as “Hillary Clinton turning up as a conservative for Halloween”.

The speculation about Mr Trump’s possible running mate has thrown up several names, including Kristi Noem and Sarah Huckerbee Sanders, the respective governors of South Dakota and Arkansas, and the conservative media personality Tucker Carlson.