AceShowbiz - Jerry Seinfeld was due to perform at a benefit for The Eric Trump Foundation, run by the mogul's son, in September, but cancelled after Donald Trump began publicly questioning President Barack Obama's citizenship.

The real estate tycoon, who is considering a possible presidential run, has been vocal about his disapproval of the country's leader and has raised doubts over whether Obama was born in the U.S.

A furious Trump fired off an angry letter to Jerry Seinfeld on Wednesday, telling him he ought to be "ashamed" for withdrawing from the gala, benefiting the St. Jude Children's Research Hospital.

Trump writes, "I just learned you cancelled a show for my son's charity because of the fact that I am being very aggressive with respect to President Obama, who is doing an absolutely terrible job as our leader."

"The Apprentice" star then goes on the attack by admitting he wished he had axed his appearance on Jerry Seinfeld's TV series "The Marriage Ref".

Trump adds, "We don't care that you broke your commitment even though the children of St. Jude are very disappointed, and despite the fact that your manager clearly stated you are 'truly a man of his word'."

"What I do feel badly about is that I agreed to do, and did, your failed show, 'The Marriage Ref', even though I thought it was absolutely terrible... Despite its poor ratings, I didn't cancel on you like you cancelled on my son and St. Jude. I only wish I did. You should be ashamed of yourself!"

Jerry Seinfeld made a contribution to both The Eric Trump Foundation and St. Jude after cancelling his slot.