President Donald Trump has branded Elon Musk a "train wreck".

The 78-year-old commander-in-chief hit out at his former friend and advisor after the tech mogul declared his intention to start his own political party, the America Party, and warned the Space X boss that the initiative would fail.

He wrote on his Truth Social platform: "I am saddened to watch Elon Musk go completely 'off the rails,' essentially becoming a TRAIN WRECK over the past five weeks. He even wants to start a Third Political Party, despite the fact that they have never succeeded in the United States."

Trump warned that Musk's plans for a third political party to rival the Democrats and Republicans would cause "total disruption and chaos".

He continued: "The one thing Third Parties are good for is the creation of Complete and Total DISRUPTION CHAOS," Trump wrote in his Truth Social post, "and we have enough of that with the Radical Left Democrats, who have lost their confidence and their minds! Republicans, on the other hand, are a smooth running 'machine,' that just passed the biggest Bill of its kind in the History of our Country."

The two men fell out because of a proposed spending bill, and Trump claimed Musk objected because of his own personal financial interests.

He wrote: "It is a Great Bill but, unfortunately for Elon, it eliminates the ridiculous Electric Vehicle (EV) Mandate, which would have forced everyone to buy an Electric Car in a short period of time. I have been strongly opposed to that from the very beginning.

"People are now allowed to buy whatever they want - Gasoline Powered, Hybrids (which are doing very well), or New Technologies as they come about - No more EV Mandate."

The Tesla boss shared his intention to set up a political party to give people "back [their] freedom" over the weekend.

He wrote: "By a factor of 2 to 1, you want a new political party and you shall have it! When it comes to bankrupting our country with waste + graft, we live in a one-party system, not a democracy.

"Today, the America Party is formed to give you back your freedom."