Sir Richard Branson has published an abusive letter he received from Donald Trump in 2004.
The two moguls have had a long-running feud, which Mr Branson expands on in his new autobiography, Finding My Virginity.
Mr Trump wrote to the Virgin Airlines founder after he launched The Rebel Billionaire: Branson's Quest for the Best, a reality TV show with a similar format to The Apprentice.
"At least your dismal ratings can now allow you to concentrate on your airline which, I am sure, needs every ounce of your energy," Mr Trump wrote in the letter.
"It is obviously a terrible business and I can't imagine, with fuel prices etc, that you can be doing any better in it than anyone else.
"Like television, you should try to get out the airline business too, as soon as possible! Actually, I wonder out loud how you can be anywhere close to a billionaire and be in that business. Perhaps the title of your show, The Rebel Billionaire, is misleading?
"In any event, do not use me to promote your rapidly sinking show - you are a big boy, try doing it yourself!"
Five days after receiving the letter, Mr Branson wrote a cordial reply in which he defended his title of 'billionaire', and advised Mr Trump not to waste his time with thoughts of revenge.
"[…] You told me you were going to spend whatever it took to get your own back on those who had not returned your calls and had not helped you when you were near bankruptcy.
"I believed it was a waste of your talent and energy and is not the best advice to give budding entrepreneurs."
Mr Branson published a blog post on his website shortly before the 2016 presidential election, in which he urged Americans not to vote for Mr Trump.
In the blog he described a past meeting with Mr Trump that left him "disturbed and saddened", after the future President spent the entire time talking about his plans to "destroy" five people who had not lent him money.
"What concerns me most, based upon my personal experiences with Donald Trump, is his vindictive streak, which could be so dangerous if he got into the White House," Mr Branson wrote.
"For somebody who is running to be the leader of the free world to be so wrapped up in himself, rather than concerned with global issues, is very worrying."
Mr Branson told Newshub in May that Trump's first months in office were "an embarrassment to the world".
"The first days in office have been so disastrous that I think the chances of it lasting more than one term are extremely unlikely and I think that's the only saving grace about the way he's behaving at the moment."